Font standardisation

[CarlosNZ]

Tidy up font implementations to make them easier to use:

• a handful of font styles defined in theme (headers, body, table, etc) and use them throughout, and only apply additional override styles inline for special cases
• suggest using rem units for all font-sizes
• etc?

[mark-prins]

Also #517
Also #641

[CarlosNZ]

I'll have a play with this tomorrow -- I'll create a small set of named styles (Header, Header2, body, bodyAlt, something like that) and check them with you before implementing them everywhere. I'll also try and make sure text elements are mapped to h1, h2, etc as much as possible so they're semantically sensible in the HTML.