This re-implements the auth store with pinia.

Other notable changes:
* the OIDC client configuration and setup is now part of
  separate module that can be included by the OIDC callback routes
  and ensures that we always use the same OIDC client configuration.
* Authorization headers for the steering and tank REST APIs are now
  set with an axios interceptor instead of specifying them for each
  API store action.

refs #127

The show selection mechanism relied on the parent component to notify
other components, even though the selected show is already part of the
store and all component state can and should be derived from there.

Users should always be able to add new shows even if none have been
created thus far.

refs #97

This is a stopgap measure until we’ve worked on #17 [1] and added a
management interface for simple models like funding categories and show
types.

refs #97

[1] #17

Up until now we had two different show selectors based on the number of
shows that were available. We now use the same show selector for any
number of shows.

Apart from that shows are now sorted
 1. based on their active state (descending, so inactive last) and
 2. their name (ascending, so A-Z)
in the show selector so it’s easier to find relevant shows.

Other changes:
* title handling has been moved into PageHeader component
* calendar management has been removed and is now handled
  locally in the EmissionManager component
* inactive shows are badged

refs #122

Tank only returns an entries property for playlist objects if the
entries list would have at least one item. This is a bug [1].

closes #119

[1] tank#46