[feature] Add image descriptions for default avatar + header; don't allow editing default desc

[tsmethurst]

Description

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This pull request adds image descriptions for the default avatars + headers used when no explicit avatar or header is set. Also adds avatar_media_id and header_media_id to account model, and uses that on the settings panel to determine whether or not a non-default avatar/header has been set. If not, the image description field for header/avatar is disabled.

Closes #3452
Closes #3453

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 not leveraged AI to create the proposed changes.
• 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.