Show an alert when trying to preview while uploading media

[yaelirub]

Fixes #13929

To test (Classic and block editors - Please test on device):

1. Go to My Site -> Posts then select a Post to edit.
2. Add an Image / gallery block or classic
3. While the image being uploaded, Tap the ... menu at the top-right corner, select Preview
4. See the alert if image still uploading
5. When Image done uploading, tap preview again
6. See preview with the image

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[leandroalonso]

@yaelirub it worked smoothly in Gutenberg. However, I had a problem with Aztec: https://d.pr/v/98bx05

For some reason, the "local path" remains (something that was not introduced by your PR — it's there for some time).

Do you want to check that or shall we merge this one?

[yaelirub]

I saw it and I think there's an issue for it.
AFAIK, out of scope for this PR