Enhancement: Refactor runtime boot process

We have updated the way applications are being loaded in 
the web runtime. It does now feature a dedicated boot process, 
providing hooks that other applications can take advantage of.

https://github.com/owncloud/web/pull/5752
https://github.com/owncloud/web/issues/2891 (tbd)
https://github.com/owncloud/web/issues/3726 (tbd)
https://github.com/owncloud/web/issues/3771 (tbd, "needs api tweak")
https://github.com/owncloud/web/issues/4735 (tbd)
https://github.com/owncloud/web/issues/5135 (tbd)
https://github.com/owncloud/web/issues/5460 (tbd)
