feat: Add dataset tagging to the back-end

[cccs-Dustin]

SUMMARY

In the current version of the master branch, the back-end only supports tagging for 'Charts', 'Dashboards' and 'Saved Queries'. However, this PR would add the back-end support for tagging 'Datasets' as well. This is just the first of at least 2 PR's, the second of which will introduce new API endpoints as well as add dataset tags to the front-end.

This PR also incorporates a few changes to files for refactoring purposes:

1. The model for tags was moved from superset/models/tags.py to superset/tags/models.py (all of the import statements have been updated to point to the new location). This was done so that going forward, the model will be in the same location as the API and command code, which will be added later on.
2. The SQLAlchemy event listeners for adding, updating, and removing tags have been removed from each model, and were instead placed into a singular location: superset/tags/core.py. The event listeners are now registered from within the init_app_in_ctx method in the superset/initialization/__init__.py file, and are placed behind a check for the TAGGING_SYSTEM feature flag.

NOTE: If you are not starting from a 'fresh' database, be sure to run the following command: superset sync_tags. This will add tags to all of the existing datasets.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

To manually verify the changes, first connect to the local database, then run the following:

Before Code Changes:

1. SELECT * FROM tag; (you should not see a dataset entry)
2. SELECT * FROM tagged_object; (you should not see any entry with the dataset tag/type)

After Code Changes:

1. SELECT * FROM tag; (you should now see a dataset entry)
2. SELECT * FROM tagged_object; (you should now see some entries with the dataset tag/type)

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags: TAGGING_SYSTEM
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #20892 (3611579) into master (b739e27) will increase coverage by 0.01%.
The diff coverage is 53.04%.

@@            Coverage Diff             @@
##           master   #20892      +/-   ##
==========================================
+ Coverage   66.66%   66.68%   +0.01%     
==========================================
  Files        1793     1794       +1     
  Lines       68492    68562      +70     
  Branches     7277     7277              
==========================================
+ Hits        45663    45722      +59     
- Misses      20967    20978      +11     
  Partials     1862     1862              

Flag	Coverage Δ
hive	52.89% <15.65%> (-0.19%)	⬇️
mysql	78.21% <53.04%> (+0.01%)	⬆️
postgres	78.28% <53.04%> (+0.01%)	⬆️
presto	52.79% <15.65%> (-0.19%)	⬇️
python	81.41% <53.04%> (+<0.01%)	⬆️
sqlite	76.78% <53.04%> (+0.01%)	⬆️
unit	50.91% <14.78%> (-0.15%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/common/tags.py	0.00% <0.00%> (ø)
superset/models/dashboard.py	76.80% <ø> (+0.72%)	⬆️
superset/models/slice.py	86.20% <ø> (+1.11%)	⬆️
superset/models/sql_lab.py	76.51% <ø> (-0.38%)	⬇️
superset/views/tags.py	48.93% <40.00%> (-1.07%)	⬇️
superset/initialization/__init__.py	91.46% <66.66%> (-0.26%)	⬇️
superset/tags/models.py	88.42% <90.90%> (ø)
superset/models/core.py	90.60% <100.00%> (+0.41%)	⬆️
superset/tags/core.py	100.00% <100.00%> (ø)
superset/tasks/cache.py	61.15% <100.00%> (ø)
... and 8 more

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[AAfghahi]

Could you add some unit tests to make sure that the commands are working as expected.

[cccs-Dustin]

Closing this PR so that I can add unit tests to it. I will create a new PR once that is done.

[villebro]

LGTM! 👍 One minor comment regarding the tagging system fixture, but I don't consider it blocking, as flipping a default feature flag will always require some test refactoring anyway.

[dpgaspar]

Looks good!
One question, why the move of /superset/models/tags.py to /superset/tags/models.py?

Note: We are currently moving and deprecating all endpoints from /supeset to /api/v1

The requested changes have been made

[villebro]

Could you add some unit tests to make sure that the commands are working as expected.

FYI @AAfghahi I removed the change request from your review as the requested tests have been added to the PR