List all changes related to a build.

 - to do this, the column 'parent_changeid' was added to the table changes

Note:
 - I added a tab 'changes' for the build webpage. I think this should be modified in order to have something more readable (but my angularJS skills are very limited).
I agree with @tardyp, but I don't think we should replicate the whole commit tree in the DB. What we need to do is just the following:

Store all the changes that are responsible for a sourcestamp in the scheduler.

In case of collapsed buildrequest: get all the changes for all the sourcestamps of all the subbuildrequests.