FIX: Change request method for categories/search (#26976)

This commit changes request method for "categories/search" from GET to
POST to make sure that long filters can be passed to the server. For
example, category selectors with many categories are setting the full
list of selected category IDs to ensure these are filtered out from the
list of choices. This can result in a long URL that exceeds the maximum
length.

lib/read_only_mixin.rb

@@ -52,6 +52,7 @@ module ReadOnlyMixin
 
   def block_if_readonly_mode
     return if request.fullpath.start_with?(path "/admin/backups")
+    return if request.fullpath.start_with?(path "/categories/search")
     return if request.get? || request.head?
 
     if @staff_writes_only_mode