Use flexbox for positioning the Boost footer

Please test this on all supported browsers, see  https://docs.moodle.org/dev/Moodle_3.5_release_notes#Browser_support This is a change for theme boost only Test a page without blocks Log in as Admin Use a large monitor and use the full screen for your browser or use your browser controls to zoom out. Browse to the "My Private files" page Resize your browser or zoom out so everything fits within the viewport, verify you do not need to scroll to see the footer. Test a page with blocks Browse to a course with blocks Resize your browser and verify the footer is fully visible without having to scroll until there is not enough space and the entire page scrolls. Test a secured page Create a new quiz On the quiz editing form scroll down to "Extra restrictions on attempts", click the show more link and select "Full screen popup with some Javascript security" Save the form an add a question of any type Attempt the quiz Resize your browser or zoom out so everything fits within the viewport, verify you do not need to scroll to see the footer. Note: This footer is expected to be just a single strip, with no contents. Test a maintenance page Log in as Admin Purge caches Edit main version.php and increase $version by .01 Go to /admin/index.php in your site and upgrade should begin. Verify that the pages shown in the upgrade process (plugins, environment, upgrade process...) display the footer properly and the page content doesn't overlaps with it. Installation Install moodle via web, from scratch, without config.php so the complete installer will be executed. Verify in all the install pages, both the "pre-installer" ones (those in charge of creating the config.php file, and the installation ones (environment, site settings, admin user creation...) that the footer is displayed properly, after the content and the content never overlaps it.