Bug fix for assert_template when opening a new session.

See https://github.com/rails/rails/pull/16234#commitcomment-7115670.
Improve description of tests.
Add support for Postgresql JSONB

[Philippe Creux, Chris Teague]
adding indexes on tables in migration scripts

missed one migration script in last commit

remove some empty lines

using the belongs_to index option to be more concise
Merge pull request #16272 from tgxworld/fix_template_assertion_when_opening_a_session

Fix template assertion when opening a session.
Merge pull request #16220 from pcreux/postgresql-jsonb-support

Add support for Postgresql JSONB
Merge pull request #16258 from mattwarrenrnp/master

adding indexes on tables in migration scripts [ci skip]
docs, add ref where to find valid `head` status symbols.

[ci skip]