Increase size of other_notes field

This limit was somewhat abitrary and was included to deter people
from writing too much.

However, people want to write more. Increase the limit.
Merge pull request #266 from alphagov/increase-text-field-size

Increase size of other_notes field
Merge pull request #265 from alphagov/bugfix/bad-request-on-nested-module-error

Use an HttpResponseBadRequest to return HTTP error