Fix custom markings

[chisholm]

The main fix in this PR is for custom markings. The changes which had been made to @CustomMarking to support the new extension-based custom markings are backed out. The new way to do this is to create a normal extension via @CustomExtension and use the extension with a marking definition, the same way as for any other object. @CustomMarking is now only used for the old-style custom markings.

Other misc changes include:

• Update the extensions jupyter notebook to correct documentation w.r.t. the above changes
• Update unit tests for custom markings w.r.t. the above changes
• Update extension registration to check toplevel property names in the same way as nested property names
• Add a unit test for an interesting oddball case where an extension can enable itself. The library supports this for now.

[codecov-commenter]

Codecov Report

Merging #521 (295037f) into master (10956d3) will increase coverage by 0.02%. The diff coverage is 94.00%.

@@            Coverage Diff             @@
##           master     #521      +/-   ##
==========================================
+ Coverage   87.20%   87.22%   +0.02%     
==========================================
  Files         153      153              
  Lines       17911    17925      +14     
==========================================
+ Hits        15619    15635      +16     
+ Misses       2292     2290       -2     

Impacted Files	Coverage Δ
stix2/v21/common.py	46.42% <0.00%> (-5.19%)	:arrow_down:
stix2/custom.py	87.34% <66.66%> (+1.79%)	:arrow_up:
stix2/registration.py	78.66% <83.33%> (+0.58%)	:arrow_up:
stix2/test/v21/test_custom.py	100.00% <100.00%> (ø)

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