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`
This fixes a TODO in the code to validate the RedirectURIs when adding
or editing an OAuth application in user settings.
This also includes a refactor of the user settings tests to only create
the DB once per top-level test to avoid reloading fixtures.