Follow social network profiles

As a User I want an easy way to start following people's social network profiles so that I can receive their feeds.

Acceptance Criteria

1. Add a subscription via command specifying the target's social network and profile alias/id
2. Add a subscription via command containing a link related to the social profile (link of a post or of the social profile)
3. Add a subscription via message containing a link related to the social profile (link of a post or of the social profile)