Refactor how data is shared by API, systems, and entities

[bachya]

Describe what the PR does:

This PR is way larger than it should be, FYI, but it was necessary to accommodate the scope of the changes I wanted.

I recently came to the conclusion that the interdependencies between the API, the system, and the entity were a mess: copies of data were being shared all over the place, it wasn't clear which object was responsible for what, and I felt anxious every time I had to look at it. 😆 So, this PR unwinds things and makes data responsibilities clearer:

• The API is responsible for subscription data. System and entity properties/methods that need this data have references to it in the API object, meaning that updates to the API data will automatically carry through to the necessary points in systems and entities.
• System objects are responsible for system, settings, and entity data. It, too, shares this data appropriately with entities so they can be automatically updated upon data updates.
• Cameras are split out from entities, as they don't have enough in common to suggest inheritance.
• Adds camera documentation that was missed in Added support to expose camera and doorbell data #182.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md and docs/ with any new documentation.
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #193 into dev will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              dev      #193      +/-   ##
===========================================
+ Coverage   99.84%   100.00%   +0.15%     
===========================================
  Files          12        13       +1     
  Lines         650       685      +35     
===========================================
+ Hits          649       685      +36     
+ Misses          1         0       -1     

Impacted Files	Coverage Δ
simplipy/api.py	100.00% <100.00%> (+0.73%)	⬆️
simplipy/camera.py	100.00% <100.00%> (ø)
simplipy/entity/__init__.py	100.00% <100.00%> (ø)
simplipy/entity/factory.py	100.00% <100.00%> (ø)
simplipy/lock.py	100.00% <100.00%> (ø)
simplipy/sensor/v2.py	100.00% <100.00%> (ø)
simplipy/sensor/v3.py	100.00% <100.00%> (ø)
simplipy/system/v2.py	100.00% <100.00%> (ø)
simplipy/system/v3.py	100.00% <100.00%> (ø)
... and 1 more

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[codeclimate]

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