Revert "MDL-36002 core: Make drag and drop keyboard friendly"

This reverts commit 7c271b9, 313e585, ade4695, and 70b4027

Conflicts:
	theme/bootstrapbase/style/moodle.css
MDL-41200 filepicker: change header string to select.
Add length limitation for header, so it fits nicely in mobile devices
MDL-41200 Filepicker: Reduce the string length to fit the dialog size nicely.
MDL-39851 javascript: Fixes for filepicker dialogs

Fixes display issues with the filepicker dialogs that were
converted to M.core.dialogue.

Note: These dialogue use table based layouts and the table width is not
counted in the form that it sits in. I have changed the dialogues to have
a wider width but for really small screens this causes horizontal scrolling.

The only solution for that is to rewrite all the filepicker/filemanager dialogs
to use css for layout.
Merge branch 'MDL-41200' of https://github.com/rwijaya/moodle
MDL-4908 Forum: Correctly check for appropriate login on maildigest changes
MDL-4908 Forum: Basic maildigest setting tests