make things resizable

[miserman]

It would be nice if at least the main output elements (like plots and maps) were user-resizable.

Stipulations

• there should be a way to reset to the initial state
• this shouldn't get in the way of other interactions

Implementation

• would probably want to make this an option on page_section elements, which can be toggled through R (i.e., allow section to be user-resized)
• might want a global toggle, so resizing could be turned on just for resizing, and turned off to avoid accidental resizing
• would need a way to track the size (original and adjusted), so it could be reset (revered to original, or set to the user-defined size on reload)