[#2301] Proper handling of User.is_discoverable

Closes #2301 (closed), fixes a bug introduced in 41939e31 (included in 2.2 release).

As in Mastodon, discoverable flag never meant to restrict users from appearing in search results.

In Mastodon this flag enables listing in profile directories (Masto feature that Pleroma doesn't currently have).

In Pleroma it controls noindex, noarchive metadata for search engine bots and other external agents.