2582: handle nested DFXP tags in the new editor

Re-wrote dfxpToMarkdown to make it work with nested tags.  Made
dfxpToMarkdown a module-level function, rather than a function of
AmaraDFXPParser.

Made a new test script with tests for dfxpToMarkdown.  Moved the old
one out of the way, since many tests were failing because of errors in
the test code.
Merge branch 'staging' into i-2564
2587: Fix resolve_ownership method

resolve_ownership should handle VideoURL objects having bad data in
account_username, but it was failing because the try..catch had bad
indentation.  While I fixed that, I also cleaned up the method a bit
and also made it so that it fixes the data in VideoURL if it can.

Replaced videos.tasks._add_amara_description_credit_to_youtube_vurl
with a mock function.  There's no reason that we need to run this task
in the tests.
2564: fixed video debug sync to YT functionality
Merge branch 'i-2564' into staging
accountlinker unittest fixes

Made the tests pass, they were failing on my vagrant box and I'm
pretty sure they would fail on the staging server as well.

Made the tests use factory functions instead of fixtures.

Added function in test_factories to create a ThirdPartyAccount
Merge pull request #211 from pculture/i-2587

I 2587
Merge pull request #212 from pculture/i-2582

2582: handle nested DFXP tags in the new editor