Classifier: add a loading state

[eatyourgreens]

• Add a loading state to the classifier container. Defer rendering the Classifier until after the container is mounted and the store has initialised.
• Fix a small auth bug in the UPP store. Get an access token after checking the current user session, before fetching user preferences.

#2863 re-opened #2954 on staging, because the classifier store now loads synchronously (loading in ClassifierContainer is never false.) This PR should fix that by deferring the first render of the Classifier until after the classifier store is initialised (and the classifier UPP store has been reset.) To test it out, load the classifier while logged out, then log in on the Classify page of a project so that you are signing in to an existing classifier store (with UPP loading state already set.)

Package:
lib-classifier

Closes #2954.

Review Checklist

General

• Are the tests passing locally and on Travis?
• Is the documentation up to date?

Components

• Has a storybook story been created or updated?
• Is the component accessible?
  • Can it be used with a screen reader? BBC guide to testing with VoiceOver
  • Can it be used from the keyboard? WebAIM guide to keyboard testing
  • Is it passing accessibility checks in the storybook?

Apps

• Does it work in all major browsers: Firefox, Chrome, Edge, Safari?
• Does it work on mobile?
• Can you yarn panic && yarn bootstrap or docker-compose up --build and app works as expected?

Publishing

• Is the changelog updated?
• Are the dependencies updated for apps and libraries that are using the newly published library?

Post-merging

• Did the app deploy to https://frontend.preview.zooniverse.org/projects/:project-name/:owner or https://frontend.preview.zooniverse.org/about?
• Is the new feature working or bug now fixed?
  • Is there a Talk or blog post written to announce the new feature(s)?
• Is the design working across browsers (Firefox, Chrome, Edge, Safari) and mobile?
  • Is this approved by our designer?
• Is this ready for production deployment?

[eatyourgreens]

I'm not sure if this can be reliably reproduced, but I was seeing this occasionally return an empty string, even when I was logged in.

[eatyourgreens]

This is a quick fix for the loading state bug. #2940 adds classifier hooks to fetch user data, and that might also be a better long-term solution for fetching project preferences.

[goplayoutside3]

PR Review

Looks good! When I navigate to a project where I've already seen the tutorial such as TESS, sign-in, and the tutorial modal does not appear. When I visit a project I have not seen before with my user account, the tutorial modal appears.

Additional testing of a random project like i-fancy-cats:

• I can sign-in and sign-out
• Make a classification
• Load all FEM project pages