R1:a9f2c07345d0

Generate a 403 page with a nice dialog when a file token is invalid

Summary:
Ref T5685. Currently we just 403 on an invalid token, but we can be a little more helpful.

The issues here are:

- If we **do** redirect you on this page and something goes wrong, you might get stuck in a redirect loop.
- If we **don't** redirect you, copy/pasting the link to someone (or reloading the page) gives them a pretty confusing result, since the link doesn't work any more. Prior to this diff, they get a 403.

To…