hide placeholder text when files have been added
vertically center dropzone area content
close modal after file upload succeeds

Fix
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10602#note_27871752

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/30637

Closes #30531

Sketch 43 files are technically a zip file, so the JavaScript opens the
zip file & locates a preview.png which is just a quick preview of the
last sketch page edited. After that is loaded it simply places the image
into the DOM

Now you can create a new file and select a target_branch != source_branch.
If the file that you want to create already exists on the target branch an error message is shown
A glDropdown is used to select and create a new branch instead of a text field.