Users aren't encouraged to follow us when they connect an elsewhere account

[Changaco]

For example when a user connects a Twitter account to their profile we should show them a message with a link to @liberapay. Same for Mastodon (main account and alt account) and Facebook (Liberapay page).

For GitHub we could link to the Liberapay organization and ask users to star the repos they like.

We don't need to try to detect whether the user already follows us or not to determine if we should show the message, that seems like unnecessary complexity.