Import projects should allow you to overwrite an existing project

**Is your feature request related to a problem? Please describe.**
When importing a project with a name collision with an existing project you are prevented from executing the import unless you rename the target.
**Describe the solution you'd like**
It would be nice to have an option to overwrite the current project (with appropriate warnings)

**Describe alternatives you've considered**
Just renaming things - annoying because _in some use cases_, the imported project is the "latest version", so there's no need to have the old project sitting around.

**Additional context**
One use case I've encountered is distributing projects to coworkers. If they import my export once, and then later on I add some data sources, they can't import the project export I send them, without renaming the target name. Even if they do this, it isn't ideal, because they'll be left with a project that contains outdated/incomplete information.

The issue names no files or tests; begin at the project import UI and trace how existing-name collisions are handled. Define the overwrite flow and warning behavior from the request, then verify that importing a newer export replaces the existing project without requiring a rename.

Written by the indexing model from the issue text.