Allow for elements selection in translation screen

When translating an article that has media elements, such as a Jetpack gallery, it would be very handy to have the option of selecting the object on the original post so that it could be pasted into the new article.

No files, tests, or entry points are named. Start by locating the translation screen and how article media elements are represented and selected; done means a translator can select a media object from the original article and paste it into the translated article.

Written by the indexing model from the issue text.