Allow users to subscribe to topic tags in a similar way to how groups can be subscribed to.

As suggested here: https://tildes.net/~tildes/aas/isnt_the_number_of_groups_too_restrictive#comment-2kf9

This function could be a nice way to allow topics with those specific tags to show up on users' front pages even if they aren't subscribed to the particular group the topic was submitted to. It would also help support niche subjects that don't quite have enough topics being submitted to justify having their own group yet.

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This feature was also discussed in ~tildes unofficial weekly discussion #2 and several more related suggestions were made.

• Concatenation. The ability to subscribe to results where a topic only has tag A and B both: https://tildes.net/~tildes/gc7/unofficial_weekly_discussion_2_topic_tag_system_brainstorming_session#comment-3q8n

• Subscribed tag colour highlighting, so they stand out on the front page: https://tildes.net/~tildes/gc7/unofficial_weekly_discussion_2_topic_tag_system_brainstorming_session#comment-3qa9

• Add more robust/granular options for tag subscriptions, so which groups/users the subscribed tags refer to can be more specific. E.g subscribe to Marlins in ~science NOT ~sports, weekly.recurring ONLY from user X, etc. https://tildes.net/~tildes/gc7/unofficial_weekly_discussion_2_topic_tag_system_brainstorming_session#comment-3qbz