permissions: drop hg.create.write_on_repogroup "Repository creation with group write access" setting

Simplify permissions system and get rid of some confusing tech debt.

Before, the global 'write_on_repogroup' setting controlled what write
permission on a repo group meant.

With this change, users can create repositories in a repo group if and only if
they have write access. Write access to a repo group will now mean the
permission to create repositories in it.

Write access to repo groups must be granted explicitly. There should not be any
other reason to grant write access than to allow users to create repos. There
is thus no upgrade concerns for this change.

An admin that doesn't want users to create repos in a repogroup should just not
give them write access.

These global settings might still exist in the database, but is ignored and no
longer used and do no harm.