Serve source view using WARC files if they exist.

This adds a Python application based on pywb that runs alongside our
Django app and serves WARC files. WARC files will be used if
link.asset.warc_capture points to a “.warc” or “.warc.gz” file. (Until
we add WARC capture, the only way that would happen is if you manually
edit the database.)

DEPLOYMENT NOTE:
This depends on forked versions of three packages that aren’t in PyPI —
pywb, warc-tools, and cdx_writer. Hopefully the changes will be merged
upstream and they’ll be added to PyPI. In the meantime they’re living
in my repo and included as git submodules.

So, when you update for the first time, you need to run:

git submodule init
git submodule update

to pull down the libraries.
Point to newer pywb.
Ignore changes in submodules.
Update version of pywb.
Update pywb.
Change cdxwriter warctools import.
started adding warcprox
Merge remote-tracking branch 'jack/dev.warc_server' into jack_warc

I need the PyWB intigration
PyWB added
added warcprox to .gitmodules
removed warcprox from library and added warcprox task
warcprox + phantom intigration are buggy but working. I have not really looked at playback yet, but it needs some work.  TODO's 1. cert handling  2. proxy socket error handling 3. more robust proxy teardown 4. Task managment needs a lot of work.
iso date formats were breaking the SVG images in firefox. changed date format to a known working format.
fixes #409
playback is working
bug fixes, image previews for pdf's and failed uploads
Merge branch 'develop' of github.com:harvard-lil/perma into develop
working prototype
This is a working prototype of Perma with warc record/playback. Requieres warcprox which can be installed with pip
Clean up link dragging.

Fixes #406 and #407
fixed single-linky header issue