refs #89

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.

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

for details see the migration guide for v2.0.0:
https://github.com/bootstrap-vue/bootstrap-vue/releases/tag/v2.0.0