Create mechanism for showing "site wide tag results" instead of only returning tag browsing results from subscribed groups

Tildes.net/?tag= only returns results from users' subscribed groups, but a "site wide tag result" option would definitely be nice to have eventually, especially as Tildes continues to grow/add more groups and user group subscriptions begin to diverge more as a result. This will allow adding something like Tag browsing results from inside ~group only. [Show all groups] header+link in the ?tag= page, similar to how it's displayed in the group restricted search results.

Discussed in Unofficial Weekly Discussion #2: https://tildes.net/~tildes/gc7/unofficial_weekly_discussion_2_topic_tag_system_brainstorming_session#comment-3qa3