[FAU-411] Add campo-key field / Add form fields validation rules

[amiut]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes/features)
• [x] Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) Feature

What is the current behavior? (You can also link to an open issue here) https://inpsyde.atlassian.net/browse/FAU-411

What is the new behavior (if this is a feature change)?

• Added campo-key field for the following taxonomies: abschluss, area_of_study, faechergruppe, standort with validation rules.
• Add validation error messages both on backend and javascript (HTML5 validation).

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?) No

Other information:

[amiut]

Thanks a lot! I probably wouldn't have touched the state (disabled/enabled) of the form submit button but let the browser handle it natively. However, this is much better and feels more native than the first approach. So, let's keep this for now and await client testing. Thanks for this great work! 💪🏽

Disabling the submit button is to prevent the form from being submitted in add term form which is an ajax-form and is controlled by a javascript file from WordPress core, I think we need it to prevent the other event listener sending form data via Ajax when the form is invalid.