[UX] move contents of "Manage file display" page

[jenlampton]

This is a follow-up to #2632

The goal is to get rid of the "Manage file display" page entirely. We should move the settings that are currently on that page under a "configure" link on the File row on the usual "manage displays" page. A good example of what this would look like can be found on any other fieldable entity.

Example of configure link (closed):

Example of configure link (open):

Current settings on the "Manage file display" page:

Destination for these settings:

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Advocate: @jenlampton

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Weekly update?

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PR: ?

[jenlampton]

This is going to be particularly difficult because the "file" row on the manage displays page is not a real field, it's a psuedofield, and psuedofields aren't currently allowed to have settings.

I have two ideas for how to approach this:

1. Allow settings for psuedofields. I think this is the "better" approach because this is a feature contrib may want to use also.
2. Create a real field for "File" instead of the psuedofield.

If the #1 approach looks too difficult or ends up being too much change, I'll see if #2 is feasible instead.

[docwilmot]

A quick alternative that still removes the ugly, would be instead of a configure button there:

Put a link to this form instead

and remove the menu item for the "Manage File Display" tab.

[quicksketch]

I'm not sure this is possible without an API change. But I think this would/should be a priority item for 1.15.0

[jenlampton]

@docwilmot I looked into adding a link, but psuedofields only allow two things currently: a label and a description. We need to rework them either way, so I'll start by adding a settings form.

[jenlampton]

No updates since Sept 19, clearing milestone. Also unassigning. :(