Imported latest 6.x version from SVN development repository.

Changelog (Exhibit):
- Added some ExhibitViewsModule usage instructions to INSTALL.txt (#256548).
- Removed completed items from TODO.txt.
- Added sponsors to README.txt; removed pre-alpha notice.

Changelog (Exhibit Core):
- Include profile fields in the users data feed, and added support role-specific data feeds (#226035 by TypQxQ and jhuckabee).
- Include taxonomy terms in the nodes data feed (#255525).

Changelog (Exhibit Views):
- Auto-prepend the base table name to the user-specified label field name.