Add generous timeout to MailHandler commands

Fixes https://github.com/mysociety/alaveteli/issues/938

> elinks (from wvText) can grow huge and bring the server to its knees

elinks requires a compile-time option to crash on malloc failure, so
a timeout was the quickest win.

Set to 20 minutes across all external commands in the method to catch
any other external commands causing a problem.