middleware: move handling of permanent repo URLs to separate middleware

This is about the handling of repo URLs like '_123' for the repo with repo_id
123. The de-mangling of such URLs was spread out across multiple layers. It
fits much more nicely as a middleware layer. The code in routing and simplehg /
simplegit can thus be removed.

The base _get_by_id function was confusing - fix it by removing it. To do that,
refactor utils introducing fix_repo_id_name to replace get_repo_by_id.

We now assume in the application that we never have any extra leading '/' in
URL paths.

And while trailing extra '/' might be fine in actual URLs, they must be handled
at the routing level, not propagated through all layers. This changeset is not
really changing that.