Add custom themes

[gareththegeek]

Adds support for custom themes

Themes must be placed in their own sub-directory in client/public/themes
Each theme must have

• a theme.css which defines fonts and colours
• one or more language files en.json, de.json etc.
• an entry in `client/public/themes/index.json

[sentinelgreg]

Couple of notes in jumping to making my own themes:

• We should use more intuitive, semantic names for CSS variables.

--background-color vs. --colour-black

• It would be great to facilitate a clear cut, tight testing loop for theme makers. They can upload the files to the appropriate folder
• Some way to easily copy and paste a "template" theme directory for editing.
• readme.md docs AND template theming docs need some context. Tutorialization, examples, etc.
• lang.json files should have comments with context around where each snippet will show up.
• How do we use imported fonts? Can that be invoked in theme.css?

I'm happy to do as much of that as I can.

[gareththegeek]

Folder should not contain spaces

[gareththegeek]

Comments are not supported in json files. I made all the translation keys the same as the text in the vanilla agon theme so you can see from context where they go.