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!