RFE: Prevent user sending large text in 'contact admin' emails

Description of problem:
The Contact Admin page does not limit the text a user can send, so a user can repeatedly send a lengthy novel through the Zanata email system filling the server.

Version
3.8 (retested with changed behaviour)

How reproducible:
Easy always

Steps to Reproduce:
1. Sign in as someone
2. Click on Contact Admin in the footer
3. Enter a massive amount of text into the subject line and message body
4. Press Send

Actual results:
Admin gets a huge email

Expected results:
This should not be allowed

Additional info: