user-evaluations: server-side python code, first draft.
User evaluations: added analysis of missed synapses.

Unfortunately removed synapses are not logged, so these cannot be detected.
Fixed up user analytics for django admin page.

Still could use some more work, but at least now:
 * users are sorted by last name
 * start and end dates are choosable
 * removed one plot that was not so useful (lower left)
 * extended lower right plot (usage bouts) to fill up the vertical space
user evaluation: count synapses added by other users
after the creation of a treenode by the user under evaluation.
Pre-seed user analytics with current date and 10 days back.
Correct typo in docs.
Created new admin page for tracking user proficiency over time.
user evaluation: add plot of missed pre, post and nodes by percent.
Circuit Graph Plot: added export to SVG and CSV.

Results in files being downloaded, directly from javascript thanks to FileSaver.js.

Notice that the CSS stylesheet had to be hard-coded into the plot,
otherwise the downloaded SVG lacks a few style attributes.
Fix up export widget to properly download files
(rather than open a new window).
Export to SWC: properly download a file
rather than open a new window with the SWC text content in it.
Merge branch 'user-evaluations'