Add support for tagging collections.

[kloor]

We needed a means to organize Items, Collections and Exhibits together for different divisions of our libraries. Items and Collections have Dublin Core fields, but Exhibits do not. Items and Exhibits have tags, but Collections do not. Investigating options, adding tags to Collections appeared to be the easiest to implement. Though it does not appear that the functionality could be added via a plugin, hence this pull request. We duplicated the code from Items as much as possible.