[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
• Add generic form field validation rules.
• Add validation error messages both on backend and javascript.

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]

Closing this PR in favor of: https://github.com/RRZE-Webteam/FAU-Studium/pull/130/ after discussions about using native browser validation features (HTML5 form validation). Sorry @o-samaras that you had code review on this PR already but it went into a different direction 🙈