add drag uploader

[f2cmb]

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
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Description

• Proposal for Functional upload modal roadmap#91
• Here is a brief description of what this PR does :

Add an uploader to documents modal for new KB/

Screenshots (if appropriate):

[cconard96]

Unless this design is only going to be used for KB, IMO it should be made its own component and not put in the KB styles/code.

[AdrienClairembault]

I think the upload to tmp part should start immediately when the file is added, that's how it works on gmail for example.

Also, if I upload a file that is not allowed (a .json for example), I don't have any error and my file is just silently ignored.

[f2cmb]

Removed dead code for file verification. Add immediate uploads.

[AdrienClairembault]

Looks good functionally but I'd like to wait for @cedric-anne opinion on the technical side as this changes a lot compared to our traditional "legacy" jquery file uploader system.

If we validate it, it should indeed be move into a re-usable module so it can be used for others features as a replacement of the old jquery file uploader.