Merge: Nitweb: boost response time and some improvements

The main objective of this PR is to shorten the frontend response time.
The major culprit was that the full documentation was serialized in each response.
This PR makes nitweb return only the synopsis and lets the frontend ask for the full documentation if needed.

Also uniformizes and cleans tabs for entities:
* Moved data between tabs (such as inheritance data to the inheritance tab)
* Added missing tabs for packages and groups (http://nitweb.moz-code.org/doc/core%3Ecollection%3E/defs)

Demo: http://nitweb.moz-code.org/

Pull-Request: #2536
Reviewed-by: Jean Privat <jean@pryen.org>