prompt to rename on upload name conflict

closes CNVS-12667

test plan 1:
  - upload a file with a name and extension that already exists in the
    folder
    - ui modal should prompt user to overwrite or rename
  - should work when selecting multiple files with name conflicts

test plan 2:
  - click the + Files button
  - select a file with a name that already exists in the folder
  - dismiss ui modal by clicking upper right corner
  - repeat with the same file
    - you should get the ui prompt again the second time

test plan 3:
  - repeat test plan 1, but:
    - when uploading duplicate file, rename the file to a different name
      that also exists
    - ui modal should prompt user to overwrite or rename

Change-Id: Id78f6832c0680c2084764166b645916d04772ab4
Reviewed-on: https://gerrit.instructure.com/39418
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Sterling Cobb <sterling@instructure.com>
Product-Review: Jason Madsen <jmadsen@instructure.com>
QA-Review: Jason Madsen <jmadsen@instructure.com>