Re-add follow recommendations API (#7918)

* Re-add follow recommendations API

    GET /api/v1/suggestions

Removed in 8efa081f21 due to Neo4J
dependency. The algorithm uses triadic closures, takes into account
suspensions, blocks, mutes, domain blocks, excludes locked and moved
accounts, and prefers more recently updated accounts.

* Track interactions with people you don't follow

Replying to, favouriting and reblogging someone you're not following
will make them show up in follow recommendations. The interactions
have different weights:

- Replying is 1
- Favouriting is 10 (decidedly positive interaction, but private)
- Reblogging is 20

Following them, muting or blocking will remove them from the list,
obviously.

* Remove triadic closures, ensure potential friendships are trimmed

app/models/account.rb

@@ -127,6 +127,7 @@ class Account < ApplicationRecord
   scope :matches_username, ->(value) { where(arel_table[:username].matches("#{value}%")) }
   scope :matches_display_name, ->(value) { where(arel_table[:display_name].matches("#{value}%")) }
   scope :matches_domain, ->(value) { where(arel_table[:domain].matches("%#{value}%")) }
+  scope :searchable, -> { where(suspended: false).where(moved_to_account_id: nil) }
 
   delegate :email,
            :unconfirmed_email,
@@ -309,34 +310,6 @@ class Account < ApplicationRecord
       DeliveryFailureTracker.filter(urls)
     end
 
-    def triadic_closures(account, limit: 5, offset: 0)
-      sql = <<-SQL.squish
-        WITH first_degree AS (
-          SELECT target_account_id
-          FROM follows
-          WHERE account_id = :account_id
-        )
-        SELECT accounts.*
-        FROM follows
-        INNER JOIN accounts ON follows.target_account_id = accounts.id
-        WHERE
-          account_id IN (SELECT * FROM first_degree)
-          AND target_account_id NOT IN (SELECT * FROM first_degree)
-          AND target_account_id NOT IN (:excluded_account_ids)
-          AND accounts.suspended = false
-        GROUP BY target_account_id, accounts.id
-        ORDER BY count(account_id) DESC
-        OFFSET :offset
-        LIMIT :limit
-      SQL
-
-      excluded_account_ids = account.excluded_from_timeline_account_ids + [account.id]
-
-      find_by_sql(
-        [sql, { account_id: account.id, excluded_account_ids: excluded_account_ids, limit: limit, offset: offset }]
-      )
-    end
-
     def search_for(terms, limit = 10)
       textsearch, query = generate_query_for_search(terms)