Add not_featured_by scope to Tag

[mjankowski]

A few changes here:

• Because of how FeaturedTag model sets the tag value (doing a lookup on name in a callback), setting the tag at all in the fabricator was making it hard to work with the fabricators. Setting this to nil and passing in a name works better.
• There are two controller locations which are doing the same logic of "get tags which were used recently by this account and are not in their featured tags". They already had a scope for the first part, this adds a scope for the second part.
• I think there's a bug fix in here as well -- the previous queries were doing something like "where tags.id not IN ?", but the source of that ? was coming from the id of a FeaturedTag association. I believe it should be coming from the tag_id of the featured tag association? The new scope does this. If this is not correct I may be misunderstanding how this works.

Possible future refactor here: I believe these two uses are the only uses of these two scopes. Usage could be shortened to like Tag.suggestions_for(account) or something, which would then call these two scopes internally.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.07%. Comparing base (86500e3) to head (4e55a8a).
Report is 379 commits behind head on main.

❗ Current head 4e55a8a differs from pull request most recent head d38c5f5. Consider uploading reports for the commit d38c5f5 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #28815      +/-   ##
==========================================
+ Coverage   85.01%   85.07%   +0.05%     
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  Files        1059     1038      -21     
  Lines       28277    28128     -149     
  Branches     4538     4531       -7     
==========================================
- Hits        24040    23929     -111     
+ Misses       3074     3039      -35     
+ Partials     1163     1160       -3     

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[mjankowski]

Rebased this after #29597 (bug fix) merged. This PR is now just scope introduction to consolidate the previous logic into Tag class.