Fixed postgis test database initialization

Refs #20968. Allow querying template_postgis presence without
existing test database.
Thanks Tim Graham for the review.
Revert "Fixed #23455 -- Forced related_name to be a unicode string during deconstruction."

This reverts commit 45bd7b3bd9008941580c100b9fc7361e3ff3ff0d.
Fixed #23982 -- Added doc note on generating Python 2/3 cross-compatible migrations.

Thanks Luke Plant for the report, and Tim Graham, Simon Charette, and Markus
Holtermann for review and discussion.
Fixed #23455 -- Accept either bytes or text for related_name, convert to text.
Fixed #23405 - Fixed asking for default value for `CharField` and `TextField` if field is NOT NULL