Disallow DELETE, POST and PUT requests
Reworked routes to handle all formats

This ensures that all requests get an appropriate reponse and no errors
are generated.
Fix `DoubleRenderError`s on 404s

This was introduced in 9a8b6978a95fa25cc2a8f3503927c9f8e229fe46.
Fix accidentally regenerating whole history

This was caused by 9146334d738305f047586ea3aa21c2e292ae8be3 and would
regenerate repository history in each update where `HEAD` of the
repository did not change.
Move import functionality to an own module
Add some missing specs for Repository