Release 0.42

[garethrees]

• [x] Requires #6603
• [x] #6011

LATER

• [x] #6348
• [ ] #6533
• [ ] #5851
• [ ] #1155

[garethrees]

Probably want to remove syck after it's served it's purpose https://github.com/mysociety/alaveteli/pull/7471

[garethrees]

Couple of unfinished todos from https://github.com/mysociety/alaveteli/issues/6603

• [ ] #6344
• [x] Deprecate vagrant https://github.com/mysociety/alaveteli/compare/develop...deprecate-vagrant
• [x] Document Docker
• [x] https://github.com/mysociety/alaveteli/pull/7352