Add Spawner options to enable admins to spawn user sessions

[flongford]

Closes #592

Makes use of JupyterHub's extendable spawner options form to provide admin users with the ability to spawn either "Admin" (remoteappadmin) or "User" (remoteappmanager) sessions when they login.

Notes

The existing session must be closed before the options form will be shown again. For JupyterHub < 1.0.0, the only way to do this is by navigating to https://<simphony-remote>/hub/admin or https://<simphony-remote>/hub/home whilst logged in and selecting the "Stop My Sever" option.

Therefore (for now) we include the SimphonyRemoteAuthMixin class, providing a logout handler that automatically closes admin user servers upon a sign out. This has no effect on the functionality of the application since any Simphony-Remote applications will continue to run even if the JupyterHub server is shut down.

For versions of JupyterHub >= 1.0.0, we can simply configure the shutdown_on_logout option to do this instead.

[codecov-commenter]

Codecov Report

Merging #598 (1ca6c84) into master (2f6e719) will decrease coverage by 0.32%.
The diff coverage is 68.00%.

@@            Coverage Diff             @@
##           master     #598      +/-   ##
==========================================
- Coverage   95.35%   95.03%   -0.33%     
==========================================
  Files          92       93       +1     
  Lines        4305     4348      +43     
  Branches      422      429       +7     
==========================================
+ Hits         4105     4132      +27     
- Misses        140      156      +16     
  Partials       60       60              

Impacted Files	Coverage Δ
...ager/jupyterhub/auth/simphony_remote_auth_mixin.py	31.57% <31.57%> (ø)
remoteappmanager/jupyterhub/spawners.py	88.54% <81.25%> (-1.71%)	⬇️
...eappmanager/jupyterhub/auth/basic_authenticator.py	87.50% <100.00%> (+0.83%)	⬆️
.../jupyterhub/auth/github_whitelist_authenticator.py	100.00% <100.00%> (ø)
...eappmanager/jupyterhub/auth/world_authenticator.py	100.00% <100.00%> (ø)
remoteappmanager/jupyterhub/tests/test_spawners.py	92.81% <100.00%> (+0.28%)	⬆️

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