Subscription to tags

Currently, tags are not often used.
In my opinion there are way more possibility in them, we see and use.
As in #231, we are already creating virtual groups/circles, by using the tag on them.

I think, if we want the users to take the effort to tag the groups, we will have to provide more features on them.

The first use idea, that came to my mind was to notify the tanulosch members of the college circles, when a new group is created to which they are subscribed.

Currently I can think about two possible implementations of the notification system.
- send an email with the details of the group and an unsubscribe button.
- send a mobile push notification and user's can delete watched tags under their own profile.

We should provide autocomplete so there is only one variation of each topic. (`buying-apple` `apple-shopping` and `Apple-shopping` should be the same thing)

No files, tests, or entry points are identified. First clarify whether tag subscriptions, email or push notifications, autocomplete, and profile management are in scope, then define the chosen behavior and acceptance criteria before locating the relevant code.

Written by the indexing model from the issue text.