move EE-only droplab/filtered search features to ee/

What does this MR do?

Moves the following files into the ee/ folder, as they are EE-specific:

• app/assets/javascripts/droplab/plugins/custom_number.js
• app/assets/javascripts/filtered_search/dropdown_weight.js

Why was this MR needed?

Graciously brought up by @godfat to make sure EE-specific features are in the ee/ folder! 👍

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
  • Has been reviewed by a Frontend maintainer
• EE specific content should be in the top level /ee folder
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#6969 (closed)