[performance] Don't fetch avatar + header if uri hasn't changed

[tsmethurst]

Description

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

This pull request updates account dereferencing logic to compare remote header/avatar url before + after latest fetch, and only fetch + store avatars and headers if they've changed since last time we saw them.

Also does a very small performance enhancement to only instantiate a transport once and reuse it for remote calls during enrichAccount.

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.

[tsmethurst]

This seems to work rather well so far, so I'm merging it like a badass!