Modernize file detail view

Summary:
Use modern elements for file detail view.

For the "Technical Details", maybe we should implement a disclosure triangle element? I'd guess there are other cases we could use it.

This makes two small practical changes:

• We show "Delete File" even if you can't delete it; I'm going to align this properly with CAN_EDIT shortly.
• We no longer show the feature discovery hint about using "arc download".

Test Plan:
{F27179}
{F27180}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D4199