changes made by lynnyliz to reorganise the mounts page (with fixes for merge conflicts, etc)

DO NOT MERGE THIS PR!

This PR is for changes that were made by lynnyliz in https://github.com/HabitRPG/habitrpg/pull/3906, with my fixes added for merge conflicts and for a couple of bugs. Currently, this PR is just for displaying my modifications to lynnyliz's code, not for actually merging.