Update the display of audit events on plot detail

To allow more flexibility in styling I made the short_descr property of
and audit display only the action that was taken and not include the
user.

I used that flexibilty to add grouping of audits when building the
context for the plot detail page. Audits by the same user on the same
day are grouped together to avoid needlessly repeating the username and
date in the recent edits list.

I allowed for showing an an audit that would normally be displayed as
"set readonly to False" as the more humane "made the tree editable" by
adding an Auditable class method that gives models a chance to generate
a unique format string for specific fields.

Fixes #567
Fixes #568
Fix plot recent audit filter

The method which fetched plot audits was not including the plot id in
the filter, so audits for other plots were showing up on the plot
detail page.

Fixes #613
Add humanized date to plot detail history

After discussion, I took Mike T's suggestion included both the date and
the relative time.

Fixes #270
Merge pull request #617 from jwalgran/topic/plot-hist-display

Update the display of audit events on plot detail