DEV: Correct typos and spelling mistakes (#12812)

Over the years we accrued many spelling mistakes in the code base. 

This PR attempts to fix spelling mistakes and typos in all areas of the code that are extremely safe to change 

- comments
- test descriptions
- other low risk areas

app/controllers/application_controller.rb

@@ -326,7 +326,7 @@ class ApplicationController < ActionController::Base
   end
 
   # If a controller requires a plugin, it will raise an exception if that plugin is
-  # disabled. This allows plugins to be disabled programatically.
+  # disabled. This allows plugins to be disabled programmatically.
   def self.requires_plugin(plugin_name)
     before_action do
       raise PluginDisabled.new if Discourse.disabled_plugin_names.include?(plugin_name)
@@ -399,7 +399,7 @@ class ApplicationController < ActionController::Base
     @preloaded ||= {}
     # I dislike that there is a gsub as opposed to a gsub!
     #  but we can not be mucking with user input, I wonder if there is a way
-    #  to inject this safty deeper in the library or even in AM serializer
+    #  to inject this safety deeper in the library or even in AM serializer
     @preloaded[key] = json.gsub("</", "<\\/")
   end