Add spam prevention to contact form

[abibroom]

This is a long-standing problem (neither introduced nor, alas, solved by #467 changes) - a lot of messages that come via the contact form are spam.

I have vague memories of a previous discussion about showing a captcha to non-UK IP addresses, did we do that on another of our websites and find it cut out a good proportion of the junk submissions? Could we give that a try here too? Or other possible solutions?

[MyfanwyNixon]

Inbox looks like this this morning

[zarino]

@MyfanwyNixon if you had to do a ballpark estimate, how long does it take you to deal with the spam that comes in, in an average week?

[MyfanwyNixon]

Every day I 'select all' and then uncheck anything that might conceivably not be spam. That only takes a minute, obvs.
However I then open ambiguous messages - perhaps 2 or 3 - to check if they are valid (they are usually not).
Five days a week, call it 10 minutes in total.

BUT there is the additional detail of having to look at inappropriate phrases which is never pleasant first thing in the morning!

[zarino]

Useful to know the cost of it, when it eventually comes to allocating developer time to adding a fix.

In the meantime, I wonder whether more Gmail filters on some of the most common phrases could help cut it down a bit?

[dracos]

Internal discussion at https://github.com/mysociety/sysadmin/issues/1639