Related Changes: Green check marks in topic tab consider mergeability

What started in 97ce5fe (2015-04-05, Related Changes: Show submittability
in the same topic tab) as a good attempt at hinting where the problems are
within the set of changes bundled by topic, there is still a flaw in there.

The green check mark shows up, when the changes are submittable, i.e.
there are all label conditions met. The submit (whole topic) button may
still be disabled as some of the changes don't merge cleanly in a dry run.
So you may run in a situation where all the changes in the topic have
the green check mark and you still cannot submit the topic.

This change makes the green check marks consistent with the submit button
by also looking at the mergeability of each change.

By introducing such a small change however the semantics of the field
'submittable' changes for ChangeAndCommit objects to actually mean
'ready for submit', I'd take suggestions on how to rename the
'submittable' field to make it reflect this change.

Change-Id: If40dfb8b485e20735456621a3b2904968550564d
1 file changed