Issue #426: Enable config module on upgrade.
Issue #426: Changed module_enable() to update_module_enable().
Removed unnecessary update function.
Issue #471: Use attributes in maintenance-page template consistently with page template.
Issue #158: Update the README.md file for Backdrop at time of release.
Merge pull request #593 from jenlampton/471-html-attributes

Issue #471: Handle html attributes in maintenance-page.tpl.php consistently with page.tpl.php.
Merge pull request #600 from jenlampton/158-readme

Issue #158: Update the README.md file for Backdrop at time of release.
Merge pull request #582 from BWPanda/426/configuration-manager-upgrade

Issue #426: Enable config module on upgrade.