FEATURE: new fast_typing_threshold site setting (#30865)

Rename `min_first_post_typing_time` to `fast_typing_threshold` and
provide admin 4 options:
- disabled
- low - 1 second
- standard - 3 seconds
- high - 5 seconds

Related PRs:
- https://github.com/discourse/discourse-zoom/pull/112

plugins/poll/spec/controllers/posts_controller_spec.rb

@@ -5,7 +5,7 @@ RSpec.describe PostsController do
   let!(:title) { "Testing Poll Plugin" }
 
   before do
-    SiteSetting.min_first_post_typing_time = 0
+    SiteSetting.fast_typing_threshold = "disabled"
     log_in_user(user)
   end