Followup to https://codeberg.org/forgejo/forgejo/pulls/7822
* Fix signaturebox's background breaking out of hashbox and thus breaking it's rounded corners
* Fix a bug where an extra right margin was applied to signbox's avatar on pull request overview by unrelated rule
* Untangle hashbox's CSS from .label - it was not a good partnership between them. Some extra properties, some that we had to override
* Move CSS out of "repo.css" to a separate file - it's clearly not only related to repos
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8721
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
