Allow bulk activation of Crucible users

The Admin ui allows users to be disabled en masse from Crucible.
We need a way to do the reverse too - enable users to Crucible en masse.

see https://confluence.atlassian.com/display/CRUCIBLE/Upgrading+from+FishEye+to+Crucible?focusedCommentId=448037284&#comment-448037284

If an instance is configured to be 'FishEye only', then all users created will have Crucible disabled.

When an instance then upgrades to a Crucible license, all users remain to have Crucible disabled.

There is currently a 'Disable selected users from Crucible' link on the edit users admin screen. This needs to be extended to allow admins to also enable Crucible users in bulk.