fix link to user files when newfiles is turned on

fixes: CNVS-16393

/dashboard/files did the same thing as /files
in old files. we don't want to continue to support
the /dashboard prefixed url and prefer to just
use /files.

test plan:
* with newfiles turned off:
* click your name in very top (next to "inbox settings logout")
* click the "Files" tab on the left
* it should render the oldfiles browsing interface

* now, with: User.find(<your_user_id>).account.enable_feature!(:better_file_browsing)
* do the same thing, and it should render newfiles

* they both should go to /files. not /dashboard/files

Change-Id: Ic18e6d93031b9ebc6798849cadc441c2a664e1f0
Reviewed-on: https://gerrit.instructure.com/44337
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>