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 mitigate this, do a little better than a bare 403: give them a link to auth and generate a new URI for the file.

If this is still confusing, the next best thing I can come up with is something like this:

- Put some modulous of the timestamp in the URI.
- If the current time is within 2 seconds of the generation time, show this dialog.
- Otherwise, redirect.

That seems like it would be okay, but I worry that "2" has to be small (so links you copy/paste -> chat -> click still work) and a small value means that a small amount of clock skew breaks things. We could use the database clock, but ehhh.

Other ideas:

- Put a hash of the remote IP in the URI, redirect if it doesn't match. Fails for companies behind a NAT gateway but should work in a lot of other cases.
- Just redirect always, there's no reason it should ever loop and browsers don't really do anything bad when there's a loop (they'll show an error after too many redirects).

I'm leaning toward letting this stabilize in the wild for a bit, then trying "always redirect".

Test Plan: {F188914}

Reviewers: btrahan, 20after4

Reviewed By: 20after4

Subscribers: epriestley

Maniphest Tasks: T5685

Differential Revision: https://secure.phabricator.com/D10215