Length limits for feedback form

[osma]

Description of the enhancement

There should be limits to the length of data processed by the feedback form, for example to combat DoS attacks.

This was already implemented for Skosmos 2.x in PR #1557 by @rvdwxl, but in that PR the limits for the "name", "email", and "subject" fields were hardcoded to 255 characters. Maybe the limit could be even shorter and perhaps configurable? And what about the main textarea, should its length be limited as well?

Who are the users that would benefit from the enhancement and how?

See #1557 - apparently there is a need for this since a PR was contributed by ArchiXL.

What new functionalities would the enhancement make possible?

Better security and robustness

Why is the enhancement important?

Security and robustness are important.