[api] fix some problems noted in production

- the buildevent can even more payload
- the request events need a "The one who did it."
- avoid crash in request show with issues
[webui] make parameters optional for save_comments

I wonder why this was never hit by the test suite ;(
[webui] restore display of last declined request in rdiff
[backend] drop the inode number from the deltaid hash, it breaks if the tree is copied

The inode is good for internal stuff, but bad for deltas, as deltas
get published and a redone delta is newly signed and thus end up
different than the old delta.
Copy over the old delta if we have one to ease migration.
[webui] Implement reverse owner "search" on the users home page
[webui] Catch the case where no OBSRootOwner is set