Limit the number of people that can be followed from one account (#8807)

Configurable soft limit of 7,500, and above that, configurable
ratio of 1.1 * followers, controlled by:

- MAX_FOLLOWS_THRESHOLD
- MAX_FOLLOWS_RATIO

Fix #2311

app/workers/import_worker.rb

@@ -37,6 +37,8 @@ class ImportWorker
   end
 
   def import_rows
-    CSV.new(import_contents).reject(&:blank?)
+    rows = CSV.new(import_contents).reject(&:blank?)
+    rows = rows.take(FollowLimitValidator.limit_for_account(@import.account)) if @import.type == 'following'
+    rows
   end
 end