[1.6.x] Fixed #14638 -- Clarified model Field.to_python() docs.

Thanks Anubhav Joshi for the patch.

Backport of b6aa60f425 from master
[1.6.x] Fixed #5726 -- Documented that inspectdb doesn't detect defaults.

Backport of 15cafaa565 from master
[1.6.x] Fixed #4991 -- Emphasized XSS ramifications of help_text not being escaped.

Backport of 5dbe2a9431 from master
[1.6.x] Fixed #23282 -- Corrected inheritance and reverse relations example.

Thanks knowledgepoint-devs for the report and claudep for review.

Backport of 7006187064 from master
[1.6.x] Fixed #17272 -- Documented that non-editable fields are skipped during model validation.

Thanks mitar for the report.

Backport of c587bd5a94 from master
[1.6.x] Fixed #23314 -- Tempered recommendation of storing profile data on custom user.

Thanks gavinwahl for the report.

Backport of 3569536491 from master
[1.6.x] Fixed #21801 -- Documented SingleObjectMixin.get_context_data() requires the object attribute.

Backport of 03c1609c47 from master
[1.6.x] Fixed #22996 -- Prevented crash with unencoded query string

Thanks Jorge Carleitao for the report and Aymeric Augustin, Tim Graham
for the reviews.
Backport of fa02120d36 from master.
[1.6.x] Fixed #17238 -- Added source code links to docs using sphinx.ext.viewcode.

Thanks santiagobasulto for the suggestion.

Backport of 9ed4a8c6b1 from master
[1.6.x] Added sphinx extension to ease generation of ticket links.

Backport of fca677fa43 from master