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.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
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