[backend] obsscheduler: implement fallback to old method
[admin] add feature to clean up sources

Delete sources older than x days but keeps at least n revisions, both
given as parameter.

Deletes sources, keeps revisions if an other revision links to it,
if this revision will be kept (because it is younger than x days or
in the count of revisions that has to be kept).
Rewrites .rev files for the projects.

Process now is like this:

- read .rev files for hashes that should not be deletet (if set)
- read in all files from treesdir to resolve hash to files
- read in all sourcefiles
- save files that should not be deleted
- remove all saved sourcefiles that are not in keepfiles array
- rewrite .rev files
- delete unneded files in treesdir
[api] remove more to_s on strings

They are of course completely harmless, but I know how the copy&paste
disease spreads nonsense
[dist] update gem versions

This brings jquery-ui-rails, jquery-rails is no longer a catch all
[admin] add a small warning for the new source cleanup feature
[api] check for invalid specified review owners on review state change
[webui] Ingore maintenance incident projects in involved_projects
[dist] Fix 01ca0d2b80216684bd0fedcdab945d424e84f7f8 mysql path.
[api] allow to track issues in attributes (Fate #315168)
[api][backend] move some more configurations from options.yml into /configuration api.

values in options.yml are still read and used. But they are not changable via
/configuration api and should be remove therefore.

This includes

 - anonymous ( was allow_anonymous )
 - registration ( was new_user_registration )
 - default_access_disabled ( was default_access_disabled )
 - allow_user_to_create_home_project ( was allow_user_to_create_home_project )
 - disallow_group_creation ( was disallow_group_creation_with_api )
 - change_password ( was change_passwd )
 - hide_private_options ( was hide_private_options )
 - gravatar ( was use_gravatar )
 - download_url ( was download_url )
 - ymp_url ( was ymp_url )
 - errbit_url ( was errbit_host )
 - bugzilla_url ( was bugzilla_host )
 - http_proxy ( was http_proxy )
 - theme ( was theme )