configurable ajax urls

ignoring the fact that this whole library is incredibly ugly... this was the quickest way to work around some cross-site protection issues involving jqmodal and https. essentially will allow us to specify:

`COUCHLOG_SINGLE_URL_BASE = 'https://www.commcarehq.org/couchlog/ajax/single/'`

in settings and then the modals will finally work. this isn't great but i don't want to invest a larger amount of time figuring out something smarter for what is purely a dev facing feature