Refine groups UI - fixes #2938

[benfrancis]

Closes #2938. Supercedes #2991.

• Adds an overflow menu to the add thing button to add either a thing or a group
• Adds a dedicated add thing screen
• Refines the overflow menu design
• Refines the group "context menu"
• Refines the group UI design including fixed width groups and new icons
• Fixes the overlapping groups bug and refines scrolling behaviour

Also:

• Replaces lots of nested divs and spans with more semantic HTML
• Uses HTML form elements for forms in order to take advantage of built-in client-side validation

TODO:

• [X] Fix re-arranging groups, which got a bit broken along the way
• [x] Optimise for mobile
• [x] Make linter happy

Follow-ups:

• Refactor the "context menu" and "group context menu" as separate dialogs (they're not really menus) #3098
• Use the refined overflow menu design everywhere #3104
• Potential for abstracting out a lot of common code in further follow-ups for cleaner code

[benfrancis]

@bewee I did some work on polishing the groups UI with a view to getting it released (that was some great work you did BTW).

I've slightly broken re-arranging groups in the process (the white dotted separator doesn't appear in the right place when dragging and dropping). I'm hoping to take another look at it tomorrow before fixing any broken tests etc. and then submitting this PR for review. If you happen to have time to take a look you may figure out the problem before me!

[benfrancis]

I've slightly broken re-arranging groups If you happen to have time to take a look you may figure out the problem before me!

@bewee Never mind, I think I've fixed it.