[Cases] UI validation for total number of comment characters

[js-jankisalvi]

Summary

This PR adds UI validation for comments maximum length.
It shows error message and disables save button when the comment exceeds 30k characters while

• Adding a new comment

• Updating an existing comment

Checklist

Delete any items that are not applicable to this PR.

• Unit or functional tests were updated or added to match the most common scenarios

For maintainers

• This was checked for breaking API changes and was labeled appropriately

Release Notes

The total number of characters per comment is limited to 30000

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)

[adcoelho]

Tested and works fine 👍 Left a couple of comments.

[cnasikas]

I think we should not disable the button if the user does not touch the form, right? You have to use form.isValid to check if the user has interacted with the form. If undefined then the form validation has not been checked yet. We can do it when we address #161461.

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 0097ebf

Failed CI Steps

• FTR Configs #15

Test Failures

• [job] [logs] FTR Configs #15 / Machine Learning jobs update groups "before all" hook for "returns expected list of groups after update"

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
cases	421.7KB	421.9KB	+193.0B

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
cases	146.0KB	146.0KB	-28.0B

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	14	16	+2
securitySolution	410	414	+4
total			+6

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	15	17	+2
securitySolution	489	493	+4
total			+6

History

• 💚 Build #140965 succeeded a12a48a
• 💚 Build #140871 succeeded f91c242
• 💛 Build #140658 was flaky 403772b

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @js-jankisalvi