Group Browsing & Admin: <1> Add a new "group picker" popup (like the one for user acct picker) on all forms (search, issue create/edit, etc) and <2> Enable admins to rename an existing group

We use groups extensively and there are a few little UI improvements that could make the feature so much more useful.

1. MAKE UI FOR SPECIFYING GROUPS LIKE SPECIFYING USERS - PROVIDE PICK-LIST POPUP: When you choose the "Specify User" drop-down box option in search, an "people" icon lights up that you can click on to get a pick-list of users to choose from. I'd like the same for whenever one has to "Specify Group". I'd like this for both SEARCH and for any other forms where one has to pick a group. UI should totally match the one provided for user acct selection.

2. ENABLE ABILITY TO RENAME AN EXISTING GROUP: JIRA does not have the ability to rename groups. As organizations evolve, dept names tend to change or merge. It's tough to keep track of old names and would be nice to rename. Right now we basically create a new group and manually re-add users to it before deleting the old group. That's annoying.