Set a char limit on legal form fields, to handle abuse

[duallain]

Description

There are a few users abusing legal forms, we should make it possible to filter messages with particular content. Optimally, it should appear to the form submitter that everything worked (may be worth adding a sleep for example to not change behavior, but basically just don't send the email).

More details about the specific form and the content we can use to block the email - https://mozilla.slack.com/archives/C4XSHENBY/p1617919909062200

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Success Criteria

• [ ] An element contributing to the definition of done for the card
• [ ] Tasks should be represented here as something like "Approved designs attached to this issue"

[danielnazer]

On reflection - this might not be needed - using simple email filters is resolving the worst of this problem. I'd still like to limit the number of characters that can be submitted in the fields of the form. (The worst abuser of the form is submitting more than 100,000 words multiple times a day.) Can we set a 5,000 character upper-limit to all fields?

[alexgibson]

Updated the title of this issue to reflect the suggestion from @danielnazer