Keyboard navigation on absence page + edit dialog

[ghost]

Describe the bug

Keyboard navigation is now (commit b1bc538) possible to use for adding and deleting absence, but the behaviour is very inconsistent. The edit dialog does not work using keyboard navigation and behaves in a very confusing way when you try to use it - i.e. it opens with focus on the cancel button and if you press tab focus moves outside the dialog. To reach a date field you have to "tab backwards". The Esc key does not work properly (seems to be depending on focus).

Other accessibility issues are:

1. Empty header (
   )
2. Empty button () x 2
3. No label for form control (
   ) x 2

Expected behavior

• The tab order and possible shift of focus should be logic and consistent.
• Editing absence should be using the same page and controls as adding absence, not popup a new (different) dialog with a different way of working.
• Testing accessibility with WebAIM should result in no errors.

Additional context

Compare to #295, #350, #454