Merge: nitweb: misc enhancements and cleaning

## PR summary

* remove useless modules since angular introduction
* factorize some angular code
* factorize routes /package, /group, /module... into /doc
* display error messages
* add a loading progress bar

### Unified routes

Instead of having different prefix on the kind of mentity, URLs are no unified under the `doc/` prefix:

* http://nitweb.moz-code.org/doc/core
* http://nitweb.moz-code.org/doc/core::Array
* http://nitweb.moz-code.org/doc/core::Sequence::append

### Error messages

Unknown mentities now trigger an error message:

* http://nitweb.moz-code.org/doc/core::Sequence::append2

### Loading bar

There is now an orange (but discrete) progress bar in the top of the screen while loading ressources dynamically.

Checkout the top of the screen while loading: http://nitweb.moz-code.org/

Pull-Request: #2193
Reviewed-by: Jean Privat <jean@pryen.org>