Validate URLs for F8 upload

[ngreenwald]

Currently there's no attempt to check that the URL is valid before uploading to Figure8, which leads to problems.

@willgraf, do you have a suggestion for a good package to use? I was thinking about using validators, but you said it would be better not to add additional dependencies to the repo.

In addition to checking the final URL, we should check that the supplied string which we use to generate the URL is formatted correctly