Users should be able to remove themselves from a folder someone else shared with them

[jonwetzel]

• [ ] Change title of dialog to "Manage Sharing"
• Split dialog into two subsections: "Share My Folder" and "Leave Shared Folder"
  • [ ] "Share My Folder" should have all the existing controls except the "Done" button, which stays at the bottom of the dialog.
  • [ ] "Leave Shared Folder" should have a dropdown box for selecting a folder shared with the user, with a button "Leave Folder"
  • [ ] When the "Leave Folder" button is clicked, a confirmation box pops up asking "Are you sure you want to leave folder "\<folder name>" shared by \<owner name>?"