fix(canvas-lms): buttons & misc

[pynappo]

🔧 What does this fix? 🔧

• Reorganizes button styling to not rely on autogenerated classnames as much
  • Mixins are used for easier organization
  • Some buttons that became unstyled with recent canvas css changes will now stay styled (hopefully)
  • Many buttons now look better/react properly to hover (modals, inbox, discussions, etc)
• Revises tooltip styling
• Styles the new file upload component on assignment submission
• misc:
  • Tooltip arrows are the proper color
  • Checkboxes are styled
  • Some icons are fixed

(It's pretty late for me but if preview screenshots are wanted, or if this PR should be split into pieces, I can do that later)

another thing to resolve: how should outlines on focused buttons/links/etc like these be colored:

in canvas I think they just use blue for everything. since in this userstyle we have the extra accent color i'm not sure whether to use only @blue or only @accent.

🗒 Checklist 🗒

• I have read and followed Catppuccin's contributing guidelines.