FIX: When user has already hit bookmark limit, do not error for clear_reminder! or other updates (#12658)

We introduced a cap on the number of bookmarks the user can add in be145ccf2f. However this has caused unintended side effects; when the `jobs/scheduled/bookmark_reminder_notifications.rb` runs we get this error for users who already had more bookmarks than the limit:

> Job exception: Validation failed: Sorry, you have too many bookmarks, visit #{url}/my/activity/bookmarks to remove some.

This is because the `clear_reminder!` call was triggering a bookmark validation, which raised an error because the user already had to many, holding up other reminders.

This PR also adds `max_bookmarks_per_user` hidden site setting (default 2000). This replaces the BOOKMARK_LIMIT const so we can raise it for certain sites.

spec/requests/bookmarks_controller_spec.rb

@@ -33,9 +33,7 @@ describe BookmarksController do
 
     context "if the user reached the max bookmark limit" do
       before do
-        @old_constant = Bookmark::BOOKMARK_LIMIT
-        Bookmark.send(:remove_const, "BOOKMARK_LIMIT")
-        Bookmark.const_set("BOOKMARK_LIMIT", 1)
+        SiteSetting.max_bookmarks_per_user = 1
       end
 
       it "returns failed JSON with a 400 error" do
@@ -51,14 +49,9 @@ describe BookmarksController do
         expect(response.status).to eq(400)
         user_bookmarks_url = "#{Discourse.base_url}/my/activity/bookmarks"
         expect(response.parsed_body['errors']).to include(
-          I18n.t("bookmarks.errors.too_many", user_bookmarks_url: user_bookmarks_url)
+          I18n.t("bookmarks.errors.too_many", user_bookmarks_url: user_bookmarks_url, limit: SiteSetting.max_bookmarks_per_user)
         )
       end
-
-      after do
-        Bookmark.send(:remove_const, "BOOKMARK_LIMIT")
-        Bookmark.const_set("BOOKMARK_LIMIT", @old_constant)
-      end
     end
 
     context "if the user already has bookmarked the post" do