Create reusable plotMover.js to use on both map page and plot detail page
Allow moving plot on map page (in "edit tree details" mode)

* "Edit tree details" mode gets buttons "Move Tree" and "Cancel Tree Move"
* Return plot location in plot_accordion.html via data attributes on form element
* Moving the plot handled by plotMover.js
* All modes share an instance of plotMarker
* Show/hide marker when user selects/deselects a tree
* Pass mapManager to modes, simplifying map update after add/move tree
* Show marker differently when it's being moved (will be more beautiful in the future)