Import status indicators

[pavish]

We need clear indication of current uploads, in a prominent manner (Similar to how google drive upload works?). Design spec needs to be revised to handle this.

• Show current uploads prominently
• Indicate loading on sidebar
• Indicate users to not close the browser tab/window
• Tests

[pavish]

Blocked by #149, #293 and design spec.

@kgodey and @ghislaineguerin Please let me know if we can update the design spec to add a prototype for this, or if you have any other solution in mind. I've created this ticket, to keep track of the above mentioned requirements.

[ghislaineguerin]

The finalized spec can be found here: https://wiki.mathesar.org/en/design/specs/import-status

[kgodey]

Reopening because this issue is for the frontend work related to implementing the spec.