Finalize event creation form

- Add client-side validation to the normal fields (all the stuff we check on the backend should be checked on the frontend as well)
- Fix the issues with the drinkAction dialog

depends on #54

Resolve the dependency on issue #54 first, then locate the event creation form and its drinkAction dialog. Compare the form's normal fields with the backend checks and investigate the dialog issues described by the project. Done means frontend validation covers the backend-checked fields and the drinkAction dialog works correctly.

Written by the indexing model from the issue text.