JENKINS-7824 Allow assigning builds/tests to other users

Implements JENKINS-7824 Claim plugin: Allow assigning builds/tests to other users

Please don't merge this pull req in yet. It is effectively here for review pending the following.

1. I would like @tan9 to review the TW NLS as I have added a new string or two
2. I really feel this feature is incomplete without a doCheckAssignee validation
 * I don't know how to add a Descriptor to this action (i.e. I tried an failed). Please advise!
3. Further to above, now we are using this locally, it is very hard to use without an autoFill helper. Even if we manage to get a Descriptor, is there a general solution for auto-filling userids (given Jenkins security could be linked to arbitrarily complex and large corporate directories)
4. The original patch emailed the assignee. I deleted all this code as, though I see the point, I didn't want to force emails onto either claim or claim users. Needs a bit more discussion...