Export & Import Community Subscriptions

This issue is a feature request to provide users a very simple way for them to:

• Export a list of all the communities that they're currently subscribed-to and
• Import a list of communities to subscribe-to

I would recommend:

1. a button on the Lemmy profile page that says "copy community list" which, when clicked fills the clipboard with a CSV list of the communities (as their shorthand identifiers) that the user is subscribed-to
2. a button on the Lemmy profile page that says import community list which, when clicked, asks the user to enter a CSV list of communities.