[Feature] Add global instance CSS customization setting

[S0yKaf]

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.

Allow instance admins to add custom CSS that will affect every page of their instance.

This is done with a new CustomCSS instance setting that works pretty much exactly like the Users CustomCSS property. This custom CSS is then requested for every page load. User styles/themes take precedence over this CSS.

closes #3348

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 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.

[S0yKaf]

Contrary to what I've said in the Issue, this will also affect the CSS in the dashboard. Again, no stress for the review, I know you're all busy with the 0.17 release.

[S0yKaf]

[GIN-debug] GET    /                         --> github.com/superseriousbusiness/gotosocial/internal/middleware_test.testHeaderFilter.func1 (2 handlers)
--- FAIL: TestThrottlingMiddleware (3.22s)
    throttling_test.go:100: did not return status 429 (200) with queueLimit=512 and request=530
FAIL
FAIL	github.com/superseriousbusiness/gotosocial/internal/middleware	15.506s
ok  	github.com/superseriousbusiness/gotosocial/internal/oauth	5.872s
ok  	github.com/superseriousbusiness/gotosocial/internal/paging	0.053s
ok  	github.com/superseriousbusiness/gotosocial/internal/processing	16.454s
ok  	github.com/superseriousbusiness/gotosocial/internal/processing/account	58.451s
ok  	github.com/superseriousbusiness/gotosocial/internal/processing/admin	47.852s
ok  	github.com/superseriousbusiness/gotosocial/internal/processing/conversations	22.749s
ok  	github.com/superseriousbusiness/gotosocial/internal/processing/interactionrequests	13.797s
ok  	github.com/superseriousbusiness/gotosocial/internal/processing/media	21.014s
ok  	github.com/superseriousbusiness/gotosocial/internal/processing/polls	6.387s
ok  	github.com/superseriousbusiness/gotosocial/internal/processing/status	30.041s
ok  	github.com/superseriousbusiness/gotosocial/internal/processing/stream	6.448s
ok  	github.com/superseriousbusiness/gotosocial/internal/processing/timeline	7.412s
ok  	github.com/superseriousbusiness/gotosocial/internal/processing/user	8.663s
ok  	github.com/superseriousbusiness/gotosocial/internal/processing/workers	163.601s
ok  	github.com/superseriousbusiness/gotosocial/internal/router	0.047s
ok  	github.com/superseriousbusiness/gotosocial/internal/text	83.479s
ok  	github.com/superseriousbusiness/gotosocial/internal/timeline	36.871s
ok  	github.com/superseriousbusiness/gotosocial/internal/trans	6.882s
ok  	github.com/superseriousbusiness/gotosocial/internal/transport	19.790s
ok  	github.com/superseriousbusiness/gotosocial/internal/transport/delivery	(cached)
ok  	github.com/superseriousbusiness/gotosocial/internal/typeutils	115.179s
ok  	github.com/superseriousbusiness/gotosocial/internal/util	0.809s
ok  	github.com/superseriousbusiness/gotosocial/internal/validate	0.022s
FAIL

This is what's failing the tests but I can't seem to be able to reproduce it locally. Thoughts?

[tsmethurst]

Yes, just a flaky test I think, I'll re-run