New file box now has default file name + extension

Whenever a user creates a file or folder in the web UI, the input field
will contain a default file name, pre-selected up to the extension for
easier typing.

The purpose is mostly to prevent users creating text files without an
extension.

Fixes #6045
Moved new file extension out of the translated string
Merge pull request #6096 from owncloud/files-txtextfornewfiles

New file box now has default file name + extension