Fill in holes conveying the password down to files.

I am very puzzled that ShareRoomsCollection was not based on
PasswordProtectedCollectionBase.  It seems to need it.  Anyway, add
that (back?)

And make adds of files to the list view include the collection's password,
if any.  Viola.
Merge pull request #231 from SpiderOak/issue-230-password-protected-content-downloads

Fill in holes conveying the password down to files.