Add listing of configuration keys

[jdevera]

Description:

A new view to list the stored configuration keys with a link to load them in the manager.

This is intended to be used in environments where secrecy is not a desired feature, as discussed in issue #200. So it is disabled by default and can be turned on by setting the APPRISE_ALLOW_CONFIG_LIST environment variable to true.

It will only work when the STATEFUL_MODE is set to simple.

Related issue (if applicable): refs #200

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• Tests added

[caronc]

Perhaps just calling the environment variable APPRISE_ADMIN as this would allow one to tag other feaures with Apprise that come up over time. Today it's just the config list. Later on the Admin feature could have use for other options as well?

Otherwise, thank you very much for this PR 🙏 🚀

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.83%. Comparing base (ecc8655) to head (3cb3926).
Report is 42 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #240      +/-   ##
==========================================
+ Coverage   99.37%   99.83%   +0.46%     
==========================================
  Files           7       11       +4     
  Lines         794     1187     +393     
==========================================
+ Hits          789     1185     +396     
+ Misses          5        2       -3     

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[jdevera]

Thanks for the review, I ended up naming it (APPRISE_)ADMIN_ENABLED to make its boolean nature clearer everywhere it was used.

I also changed the test to appease Codecov :P

[caronc]

Just made some minor changes to your commit. Thank you for such a great job!