When confirming project upload with user, use custom modal

[benjiwheeler]

Note: before implementing this UX flow, get feedback from more stakeholders, including community team.

Follow-on to #4700, further resolving #3509

When confirming project upload with user, user should see the nifty custom modal @carljbowman designed, instead of the default browser js modal.

Current:

Desired:

[apple502j]

Why don't you use Alerts?

[alexwenbj]

@apple502j I think because the built-in Alert's style is not unified in different browsers.

[benjiwheeler]

@apple502j Do you mean a specific js/css library called Alerts? Or do you mean the built-in js alert()/confirm() functions?

If you mean the built-in ones, then I echo what @alexwenbj said -- we are more confident about what we're communicating to the user if we're using our own design.