Add activity stream events for tags

[seanh]

...and show them on the tag pages.

This means adding tag_activity_list() and tag_activity_list_html() logic functions, adding the templating to render tags as HTML, and adding the tag HTML into the tag pages.

What exactly should go into a tag's activity stream? As with datasets we may want a more complex SQL query, where events from objects related to the tag (datasets, users, etc.) appear in the tag's stream.

[seanh]

Done in feature-1515-activity-streams branch. Did not add activity streams for tags (as in the ticket description), instead added 'tag added' and 'tag removed' activities into the existing user and dataset activity streams.