Fix keyboard navigation trap when opening modals (e.g. the content creation popup)

Related to WCAG criteria:

• 1.3.2 criteria (level A): meaningful sequence
• 2.1.2 criteria (level A): no keyboard trap.
  https://www.w3.org/TR/WCAG20/#content-structure-separation-sequence
  https://www.w3.org/TR/WCAG21/#no-keyboard-trap

When using the keyboard only to navigate in Web UI, it is possible to open a modal (e.g. the content creation modal) but not to interact with it afterwards.

The root of the problem is because focus is not put on it. When using the mouse to click on the "create" tab, keyboard navigation works again.

This element uses paper dialog which has an autofocus property we could use to handle that.