Confirm overwrite in SynBioHub

[jakebeal]

There are times when the user will want to overwrite a version already in SynBioHub, and times when they are about to accidentally do so and should be given an opportunity to realize their mistake and abort.

When uploading, check whether you're going to overwrite any of the elements, query the user, and give links for checking out the portions that are different and would be overwritten.