Overwrite protection

[ryanmark]

EXPLANATION

Vizier does not check if it will overwrite an already deployed project. It should check and prompt the user to overwrite a previously deployed graphic.

WHAT IS THE CURRENT BEHAVIOR AND HOW SHOULD IT CHANGE

When deploying, Vizier simply uploads files to the s3 location specified in the settings. If there are existing files, those files are automatically overwritten.

Vizier should check if files will be overwritten and prompt the user to overwrite.

Bonus points: in the confirmation box, add a checkbox for Always overwrite this project. If checked the deploy will always overwrite files for that project.

WHAT IS THE MOTIVATION OR USE CASE FOR THIS NEW FEATURE

There is no central management of vizier projects so it's possible two people could create two projects with the same name and try to deploy those projects to the same S3 location, overwriting each other.