1. add `GetSiteCookieFlashMessage` to help to parse flash message
2. clarify `handleRepoHomeFeed` logic
3. remove unnecessary global variables, use `sync.OnceValue` instead
4. add some tests for `IsUsableUsername` and `IsUsableRepoName`
Change all license headers to comply with REUSE specification.
Fix#16132
Co-authored-by: flynnnnnnnnnn <flynnnnnnnnnn@github>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>