Don't flash the empty dataset page when user triggers the download

[nozomione]

Issue Number

Closes #1809

Purpose/Implementation Notes

I've updated the UI to display a loader before the redirect during the download processing to prevent the empty dataset UI flash.

@dvenprasad, See the latest UI here. Thank you!

Changes include:

• Added a new state redirecting state to pages/download/index
• Add a new prop onRedirecting to DatasetProcessModal, which will be set upon dataset processing submission

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Refactor (addresses code organization and design mentioned in corresponding issue)

Functional tests

N/A

Checklist

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Screenshots

N/A

[dvenprasad]

🐱 🐱 🐱
Looking good!