Make dashboard more mobile friendly in 5.6.x

The meta viewport tag and file upload button were deleted from dashboard theme on concrete5/concrete5@b67115966cf29846f036f979582d370cb689238c 2 years ago, but why? We can make dashboard more mobile friendly even in 5.6.x, and we can upload images from Photos in current iOS.

This commit will re-add viewport in whole dashboard and shows file upload button. Bootstrap dropdown is still does not work, but mostly It's nicer than 5.6.2 I think.