Removed unused code

- Removed unused old core CSS code as all of the relevant bits have now been ported
- Removed TinyMCE as it won't be used in the first production roll-out and needs to be updated once we start using it again anyway
- Removed category.html page
- Removed unused parseuri plugin

Was going to make this part of my listsFollowup PR, but the size of the removed code would make this PR unreviewable. Therefore, I'm submitting this first as a separate PR.