flags: raise flag limits.

The old flag limits were:

* 1 flag per day for regular members.
* 10 flags per day for Gold users.
* Unlimited flags for approvers.

The new flag limits are:

* 10 flags in the modqueue at once for regular users.
* Unlimited flags for approvers.
* Unlimited flags for users with a high enough flag success rate. If you
  have at least 30 flags in the last 3 months, and you have at least a
  70% flag success rate, then you get unlimited flags.

10 flags at once means you can have up to 10 flagged posts in the
modqueue at the same time. Because flags stay in the modqueue for 3
days, this means you can flag on average 10 posts every 3 days, or just
over 3 posts per day.

app/models/user.rb

@@ -354,6 +354,21 @@ class User < ApplicationRecord
       upload_limit.free_upload_slots < UploadLimit::APPEAL_COST
     end
 
+    def is_flag_limited?
+      return false if has_unlimited_flags?
+      post_flags.pending.count >= 10
+    end
+
+    # Flags are unlimited if you're an approver or you have at least 30 flags
+    # in the last 3 months and have a 70% flag success rate.
+    def has_unlimited_flags?
+      return true if can_approve_posts?
+
+      recent_flags = post_flags.where("created_at >= ?", 3.months.ago)
+      flag_ratio = recent_flags.succeeded.count / recent_flags.count.to_f
+      recent_flags.count >= 30 && flag_ratio >= 0.70
+    end
+
     def upload_limit
       @upload_limit ||= UploadLimit.new(self)
     end