[bugfix] Fix up error getting account avatar/header errors, other small fixes

[tsmethurst]

Description

If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.

If this is a documentation change, please briefly describe what you've changed and why.

This pull request:

1. Fixes a bug where avatars were being set as 'avatar=false' by accident, leading to them being pruned and then later not found.
2. Allows for situations where a remote header/avatar url has changed to empty, and should therefore be unset.
3. Fixes the admin media prune remote command to initialize db state and caches properly.
4. Updates media attachment select db logic to select a bunch of IDs and then select attachments from those IDs, rather than doing big ol' queries.
5. Removes some unused db relations that were causing us to make bigger queries than we needed to be.

Tested on goblin.technology and seems OK there :)

Checklist

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If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• 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.