Subscription to tags / questions

[Errorname]

We want to be able to subscribe to a specific tag or a specific question to receive notifications about changes.

In the User Profile page, we should have a TagPicker to choose which tag to subscribe to.

On each question's page, we should have a button to subscribe to the question.

Question:

• What type of changes should trigger a notification ?
• Should we send every question or have a recap every week ?