tigefa4u projects Trac

* Added the possibility to hide repositories from the repository index, including the default repository.
* Moved the default repository to the top of the "Browse Source" page, before the repository index.
* Mark aliases to the default repository as bold in the repository index.
* Hide "Browse Source" menu if no repositories are defined.
* Simplified `trac-admin $ENV initenv` to not ask for repository info, as they can be defined later using the "Repositories" admin panel. The default repository can still be passed on the command line.
* Remove automatic enabling/disabling of `trac.versioncontrol.*` components when no default repository is configured with `[trac] repository_dir`.

Closes #8536.