added schema_migrations_table_name to ActiveRecord::Base in order that the name of the schema migrations table can be configured.

consolidated test_schema_migrations_table_name tests

Added changelog entry

edited changelog

removed commented lines

removed reader

ensure the schema migrations table is reset at end of test

added entry to configuration guide

guides typo and changelog order
Merge pull request #12578 from jeradphelps/configurable_schema_migrations_table_name

Configurable name for schema_migrations table

Conflicts:
	activerecord/CHANGELOG.md