Fix history date display bug on plot detail

This looks like a copy paste bug.
Merge pull request #638 from jwalgran/fix/plot-detail-history-date

Fix history date display bug on plot detail
DRY up references to the add tree URL hash

We funnel users into adding trees from many places in an effort to
maximize user contributions. As a result, construting the url to the add
tree mode of the map page was scattered in a few different files. I
unified this as best I could by moving the hash text out to settings.py
and then forwarding it through to the templates and the client-side code,
via the templated settings.js.

Fixes #635
Merge pull request #645 from jwalgran/fix/dry-plot-url-hash

DRY up references to the add tree URL hash
Add utility function to help selection
Move plot_popup to partials/ template folder
Add buttonEnabler module

fixes #376 on github.

Takes care of enabling buttons and selectively modifying their behavior based on the different states of users logged in and permissions they have.
Fix pluralization typo

 fixes #571 on github