Various small fixes for subcategories

- Added private version of desktop/mobile compose.scss which makes the dropdown boxes slightly smaller (this solves the "disappearing dropdown" issue)

- patched ````add_categories.rake```` such that it works for me (setting user_id to -1 is not actually needed, but it's convenient when running against a new database which only has the system user)

- increased the name validation rule for subcategories to 255 as requested by @broussev

- Added the ````subcategory_topics_per_page```` setting which is referenced in https://github.com/udacity/discourse/commit/8ab6fa19b877bf241e05953b992a73ec79696edd but wasn't actually included