Proper state/error management for file ingestion

[roll]

• fixes Large files - Loading state #622

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@romicolman
Please take a look, it fully reworks ingestion data flow and logic that properly covers:

• loading states
• error handling
• blocking states (you can't click other buttons while it's ingesting)
• large files indication

There is still a minor issue with background flickering after validation is done; can't be fixed in the following issues.

PS.
This PR also includes some code cleaning:

• removes some of unused code
• unifies assets usage

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[romicolman]

Thank you @roll for working on these changes. A couple of comments:

Can we change the "Validating..." text for "Checking errors..."?
Apart form that, everything looks ok. One small change, if possible. There seems to a lot of empty space with the introduction of the loading bar on the Upload screen. Can we adjust that a little bit? If this is not possible, merge it. It's not central to the implementation.

[roll]

Thanks!

Can we change the "Validating..." text for "Checking errors..."?

Done

Apart form that, everything looks ok. One small change, if possible. There seems to a lot of empty space with the introduction of the loading bar on the Upload screen. Can we adjust that a little bit?

I think it's something for @Faithkenny to test amd create a new issue if needed. There is a thing that without this empty space the size of the dialog will be "jumping" when there is loading/messaged/etc.

[romicolman]

OK @roll. Let's implement it as it is for now.

[roll]

@romicolman
Thanks! Please use the "Approve" so it can be merged

[romicolman]

This is approved :)

[roll]

Thanks!