Dark Archive, first pass
added template logic for single link in dark archve because of robot.txt (my model field name is only slighty shorter. gross.)
fixed #317
now tracking disk usage, but summing yesterday's usage with previous sum. fixes #322
now tracking darchive counts in stats. fixes #340
Add link folders.

This commit allows links to be sorted into folders and subfolders.

* Links are added to a folder by drag and drop.
* Folders belong to a particular user; users won't see the same folders
even if they have the same links. This makes sense because an author
will want to organize the links they create very differently from the
editor who vests them. (If we eventually let editors at the same
organization see each other's links, we can also let them see each
other's folders.)
* The same folders show up under "Links" and "Vested Links." Query
whether "Vested Links" still makes sense -- we could just have "Links"
show links either created or vested by the user.
* If links are paginated, the folders show up on every page of results.
* Folders can only be deleted if they are empty.
Folder tweaks.

Fix display of link when one user has it in a folder and other doesn't;
tweak folder display.
added maintenance page. we should make it prettier and put the darchive bulb on it.
fixed type in path to log location
path to log was still wrong. fixed.
dark archive button for vested links
added in weekly page impressions
Merge branch 'develop' of github.com:harvard-lil/perma into develop
UWash correction
Merge branch 'develop' of github.com:harvard-lil/perma into develop
associate upgraded users with a registrar or vesting org, closes #305
Merge branch 'develop' of github.com:harvard-lil/perma into develop
remove wayward images, set up messages framework
we're only looking for PermaBot in robots.txt now. If that matches, obey the rule.
fixes #345
closes #344
Merge branch 'messages' into develop