adds new edit flow to the file block

[draganescu]

Description

Closes #14795
Also fixes #10521

This is a follow up to the image block edit flow update which ports the new flow to all the blocks which have media with an edit state.

How has this been tested?

For now I've only tested locally.

Types of change

New feature (non-breaking change which adds functionality)

Screenshots

[kjellr]

From a design perspective, this works great!

Should get a code review before merge though. 👍

[kjellr]

Actually — I'm am seeing the issue @talldan reported in the old thread:

The filename and file link isn't updated when adding or replacing a file using 'Insert from URL' (could just use the url).

[draganescu]

@kjellr I have fixed the issue reported by @talldan and also rebased the PR

[kjellr]

Thanks! This is looking great. The last issue I'm seeing is that the cancel button is missing for Contributor users:

(I'm also thinking we should rewrite that description text depending on the user role, but I think that's probably another issue.)

[draganescu]

@kjellr fixed this issue with no cancel link. This fix will also solve #15517 's similar problem because the issue is in the common mediaPlaceholder component, so if this gets merged first the other will display the cancel link too.

[mapk]

I just tested the file block and noticed a weird issue. I'm using FF.

Steps to reproduce:

1. Add a file block.
2. Select a file from the Media Library.
3. After selecting the file, observe that the name is not replacing the placeholder text.

[draganescu]

@mapk for me it doesn't happen in Firefox 67. What FF do you have?

[mapk]

@draganescu I just tested again using FF 67.0.1 and it's working fine. I'm not sure which version I had running when I saw that, but I know I just updated today before testing this out.

[draganescu]

#11952 changed direction so closing this as it became irrelevant.