Set LIVE_PLOT_SECRET_KEY in settings from environment

[rosswhitfield]

The LIVE_PLOT_SECRET_KEY was never set in the setting. It was only able to communicate to livedata because of a temporary bypass. This change sets the key from the environment variables.

Ref: 5923: [LiveDataServer] Change API key handling

Short description of the changes:

Long description of the changes:

Check list for the pull request

• I have read the [CONTRIBUTING]
• I have read the [CODE_OF_CONDUCT]
• I have added tests for my changes
• I have updated the documentation accordingly

Check list for the reviewer

• I have read the [CONTRIBUTING]
• I have verified the proposed changes
• best software practices
  • all internal functions have an underbar, as is python standard
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent

Manual test for the reviewer

References

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.05%. Comparing base (fc21b34) to head (8e11066).

Additional details and impacted files

@@            Coverage Diff             @@
##             next     #168      +/-   ##
==========================================
- Coverage   77.07%   77.05%   -0.03%     
==========================================
  Files          50       50              
  Lines        4616     4615       -1     
==========================================
- Hits         3558     3556       -2     
- Misses       1058     1059       +1     

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