Stop blacklisting project path

The restriction is not necessary and just creates more exception cases that can annoy users.

The blacklist was used here because of the fear of conflicting an URLs like `/groupname/issues` for two possible uses:

1. all issues of the group
2. a project called `issues`

however case 1 is impossible since all group URLs are prefixed by `/group` (`/group/groupname/issues`), except the base `/groupname` which redirects to `/group/groupname`.

Furthermore, using the `/group` prefix is sane and should not be undone in the future, or else we would have problems whenever we want to add a new path to the blacklist.

This is also the URL strategy used by GitHub.