fix(analytics): move feature_name index into its own migration file

[matthewelwell]

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Changes

This PR shuffles the migrations in the app_analytics module that were added in this PR: #3420.

This achieves point 2 in the description of the issue here.

As discussed there, this is only a partial fix that will at least ensure that migration retries will succeed once the index has been created, even if the migration process is timed out.

How did you test this code?

• Ran the 2 migrations forwards and backwards on an empty table to confirm that the happy path works correctly.
• Ran the 2 migrations forwards and backwards on a database with 1,000,000 feature evaluation records
• Ran the migrations up to and including 0002, added the index manually, then ran 0003 to confirm it handled the existence of the index correctly

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d63a289) 95.92% compared to head (6e30f03) 95.92%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3427   +/-   ##
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  Coverage   95.92%   95.92%           
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  Files        1090     1091    +1     
  Lines       33949    33954    +5     
=======================================
+ Hits        32564    32569    +5     
  Misses       1385     1385           

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