Adding template_params to datasource editor for sqla tables

[michellethomas]

If a sqla table previously had template_params, saving changes in the datasource editor will override template_params. To fix this, I'm adding the template_params field to the "Advanced" section of the table editor.

Tested by:
Created a sqllab datasource based on birth names with jinja params.
Edited the template_params using the datasource editor and confirmed that the param was getting updated.

@mistercrunch @graceguo-supercat @john-bodley

[codecov-io]

Codecov Report

Merging #6869 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6869      +/-   ##
==========================================
+ Coverage   56.22%   56.32%   +0.09%     
==========================================
  Files         527      527              
  Lines       23529    23532       +3     
  Branches     2782     2784       +2     
==========================================
+ Hits        13229    13254      +25     
+ Misses       9888     9866      -22     
  Partials      412      412

Impacted Files	Coverage Δ
...uperset/assets/src/datasource/DatasourceEditor.jsx	61.84% <100%> (+0.5%)	⬆️
superset/connectors/sqla/models.py	81.53% <100%> (+0.03%)	⬆️
superset/views/core.py	75.01% <0%> (+0.28%)	⬆️
superset/models/core.py	83.56% <0%> (+0.62%)	⬆️
superset/db_engine_specs.py	53.82% <0%> (+1.84%)	⬆️

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