[REF] Support endpoint-based URL generation in UH
[IMP] Support url generation in JS
[REF] Use build_url when creating URLs in JS

- only used for urls defined in Indico.Urls
- there may be others passed e.g. through a template
- those need to be fixed later when switching to clean urls
- trying to build incomplete urls will fail with a BuildError so they
  will be easy to spot
[FIX] Update (Optionally)SecureURLHandler
[IMP] Properly handle errors in build_url
[FIX] list params in legacy urls
[FIX] Rebuild URL on fragment change
[REF] Clean up getURL usage in UH subclasses
[MIN] Use url rules for room photo urls
[REF] Get rid of UH Build/Derive functions
[REF] Merge RHTemplateContentJS and RHGetVarsJs

There is no point in splitting it in two classes...
[REF] Use endpoints for most UHs
[REF] UH whitespace cleanup
[MIN] Remove overly verbose docstring

It was really not necessary and full of typos...
[FIX] defaultParams in optionally secure UH
[REF] Properly use Flask routing and UHs for APIs

also move api/jsonrpc routes into a blueprint
[FIX]  OptionallySecureURLHandler behavior

it needs to create non-secure urls with secure=False, even if the
current request is using https
[FIX] Build absolute URLs in build_url()
[IMP] Use blueprints for plugins
[REF] Use a blueprint in the Collaboration plugin