fix: Added maxLength and default tip for email subject

[Inferato]

If the user puts more than 128 symbols in the email subject it causes an error.
Added length restriction and field tip.
Related PR: #154

[openedx-webhooks]

Thanks for the pull request, @Inferato! Please note that it may take us up to several weeks or months to complete a review and merge your PR.

Feel free to add as much of the following information to the ticket as you can:

• supporting documentation
• Open edX discussion forum threads
• timeline information ("this must be merged by XX date", and why that is)
• partner information ("this is a course on edx.org")
• any other information that can help Product understand the context for the PR

All technical communication about the code itself will be done via the GitHub pull request interface. As a reminder, our process documentation is here.

Please let us know once your PR is ready for our review and all tests are green.

[arbrandes]

Mind linking to the PR to master in the description?

[arbrandes]

(And also linking to this PR from the one to master.)

[Inferato]

(And also linking to this PR from the one to master.)

Done

[Inferato]

@mphilbrick211 Pls recheck PR and add comments if needed

[mphilbrick211]

@mphilbrick211 Pls recheck PR and add comments if needed

@arbrandes - I think this is ready for you!

[Inferato]

@arbrandes This is ready for review.

[arbrandes]

We need to merge the one to master first, and in that one I'm still missing a screenshot with the feature being introduced.

[mphilbrick211]

@arbrandes - can this be merged now that the master is all set?

[dyudyunov]

Hi
I think that there is no sense in merging into the Palm branch.

I propose to close the PR. I will create a backport to Quince instead

[dyudyunov]

I've added a backport PR for Quince #196

[openedx-webhooks]

@Inferato Even though your pull request wasn’t merged, please take a moment to answer a two question survey so we can improve your experience in the future.