Make autosuggest for mentions return followed accounts first

This makes it so that (when elasticsearch is disabled) when a user types '@foo' in the compose box, they are first going to get accounts they follow ordered by the ranking algorithm, and then second they will get accounts they do not follow, also ordered by the ranking algorithm.

This makes behavior more consistent with user expectation and also with results when elasticsearch is enabled.

Fixes #1272

app/models/account.rb

@@ -512,12 +512,19 @@ class Account < ApplicationRecord
           LEFT OUTER JOIN follows AS f ON (accounts.id = f.account_id AND f.target_account_id = :id) OR (accounts.id = f.target_account_id AND f.account_id = :id)
           LEFT JOIN users ON accounts.id = users.account_id
           LEFT JOIN account_stats AS s ON accounts.id = s.account_id
+          LEFT JOIN (
+            SELECT target_account_id
+            FROM follows
+            WHERE account_id = :id
+            UNION ALL
+            SELECT :id
+          ) AS first_degree ON accounts.id = first_degree.target_account_id
           WHERE to_tsquery('simple', :tsquery) @@ #{TEXTSEARCH}
             AND accounts.suspended_at IS NULL
             AND accounts.moved_to_account_id IS NULL
             AND (accounts.domain IS NOT NULL OR (users.approved = TRUE AND users.confirmed_at IS NOT NULL))
-          GROUP BY accounts.id, s.id
-          ORDER BY rank DESC
+          GROUP BY accounts.id, s.id, first_degree.target_account_id
+          ORDER BY first_degree.target_account_id, rank ASC
           LIMIT :limit OFFSET :offset
         SQL
       end