Enable authentication via Github access token

[c-w]

The endpoint that registers new Lokole clients is currently protected via HTTP basic auth in client-register.yaml and the username and password is configured via a secret in Kubernetes. As we're onboarding new organizations that will deploy Lokole devices, we need an easier way to add and remove people who're authorized to authenticate with the API.

This pull request adds support for authenticating with the registration endpoint via Github identity. The process for enabling a user to authenticate is as follows:

1. An administrator in the Github Ascoderu organization adds the user to the lokole-registration team.
2. The user creates a personal access token with the read:org scope.
3. The user can now pass <github-user-name>:<personal-access-token> as the value of the registration credential argument to the install.py script to set up new Lokole devices.

Resolves #245

[codecov-io]

Codecov Report

Merging #247 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #247      +/-   ##
==========================================
+ Coverage   98.58%   98.64%   +0.06%     
==========================================
  Files          14       14              
  Lines         987     1036      +49     
==========================================
+ Hits          973     1022      +49     
  Misses         14       14

Impacted Files	Coverage Δ
opwen_email_server/services/auth.py	100% <100%> (ø)	⬆️
opwen_email_server/config.py	100% <100%> (ø)	⬆️

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