Added support for database-driven theme templates.

UX/UI Improvements

• Added default "hard" (artisan down) maintenance mode view, able to be overridden by a maintenance.php file placed in the project root.
• Added support for previewing SVG images in the Media Library

API Changes

• Added Url::buildUrl() method (along with helper http_build_url()) as a polyfill for the PECL HTTP http_build_url() function.
• Added support for database-driven theme templates, enable cms.databaseTemplates to use it

Bug Fixes

• Fixed issues with hasMany relationship management by reverting some performance PRs to that process that were causing issues.
• Fixed adding new dashboard widgets defaulting to full screen width instead of 5/6th of full screen.