Add rudimentary spam protection

The GOV.UK feedback forms get regularly spammed with the same/similar
submissions (which are usually probing for SQL injection vulnerabilities
and the like). This mechanism allows those spam submissions to be filtered out
without notifying the spammer that they've been removed.
Merge pull request #130 from alphagov/spam-detection

Add rudimentary spam protection
Service feedback: switch from support to support-api
Bump gds-api-adapters

This picks up the `support-api` interface.