Improve design of transcript pages

[wrightmartin]

• [x] Tighten up whitespace
• [x] Test on mobile devices, balance information density and readability

[wrightmartin]

Possibly drop avatar sizes - they require too much whitespace at the current size where transcripts are short, back-and-forth conversations eg. http://charles-taylor.martin.sayit.dev.mysociety.org/hearing-23-september-2008/witness-adesanya-sandy-hyde-sworn

[wrightmartin]

Will come back to this after #122

[wrightmartin]

:+1: Done