fork/copy from previous version with source and translation

http://bugzilla.redhat.com/show_bug.cgi?id=1104015

- [x] selector on version create page to choose starting content
- [x] ~~options: empty, each other version **What?** - no idea what this means.~~
- [x] this element should be prominent ~~(possible modal?)~~
- [x] copies all source documents, their translations and history **history not ~~copied~~ displayed**
- [x] copy is done as a background task of some kind
- [x] raw documents also need to be copied since they are mutable
- [x] show progress indicator on version page until copy has completed
- [x] while copy is occurring: source document upload is prevented
- [x] while copy is occurring: translation is prevented
- [x] while copy is occurring: can probably force the version to READONLY until the copy is finished (make sure REST endpoint can't push to READONLY versions)
- [x] 'copy' button on existing versions that will open the version create dialog with the version selected as the starting content - deferred: https://bugzilla.redhat.com/show_bug.cgi?id=1127066
- [x] copy can be canceled. Canceling will stop the copy, leaving any already-copied documents and translations in the new version.