Update migration docs

[rpkilby]

Small cleanup of the migration docs.

• Moved issue links into headers, added issue links for 2.x migration docs.
• Moved note about enabling python warnings to top of the doc.
• Added missing note filter .required/.widget being merged into .extra.
• Removed old strictness from settings docs.

One question - I've moved the issue links into the description, but should we just remove them?

[codecov-io]

Codecov Report

Merging #866 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #866   +/-   ##
=======================================
  Coverage   97.95%   97.95%           
=======================================
  Files          15       15           
  Lines        1122     1122           
=======================================
  Hits         1099     1099           
  Misses         23       23

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[carltongibson]

Yep. Great as ever. 👍