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

config/locales/en.yml

@@ -917,6 +917,7 @@ en:
       tips: Tips
       title: Welcome aboard, %{name}!
   users:
+    follow_limit_reached: You cannot follow more than %{limit} people
     invalid_email: The e-mail address is invalid
     invalid_otp_token: Invalid two-factor code
     otp_lost_help_html: If you lost access to both, you may get in touch with %{email}