Rework todo list page

There is a nice and detailed suggestion on how to improve the todo list page on Github. This should be used as a base to rework the layout of the todo list page and make it more user friendly.

Release Checklist (to be enhanced as required)

• Fix calendar size on small screens (for date selection)
• Cannot set due date on todos when shown in side bar
• Wrong background color in non-covered parts of todo side bar
• Page title does not break
• Back button not working in todo drawer
• Round buttons too small on (some) smartphone screens
• Improve adding new todos (and tasks)
• Close notes editor when pressing escape or back
• Close notes editor when hiding the todo drawer
• On Android, increase the margin left and right of the markdown editor, to allow easier scrolling when the text is longer than what fits on the screen.
• Check back button handling in schedule page.