[performance/postgres] Rename constraints, remove duplicate indexes

[tsmethurst]

Description

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This PR drops a lot of duplicate indexes in cases where:

• a unique constraint for columns in the index already exists (so this index is a duplicate); OR --
• a covering index already exists for the indexed column

The PR also renames constraints that were misnamed in previous migrations (Postgres only).

closes #2122
closes #2017

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[daenney]

That looks fair, as long as we're sure about the indexes we're dropping.

For posterity: relational databases automatically create indexes for the primary key as well as any column with a unique constraint. We just so happened to also be creating those indexes, duplicating a bunch of work, paying the cost for updating two indexes instead of one and using some extra space for no good reason.