[UI] Sample importing should be disabled in mixed mode

What's the issue?
Sample importing in mixed mode is unsupported (and doesn't work) so it should be disabled in the WebUI.

What's the fix?
Users shouldn't be able to load a sample bucket in mixed mode.

Steps to reproduce
1) Create a mixed mode cluster with 6.6.x and 7.0.x
2) Try to load a sample bucket from either 6.6.x or 7.0.x node

Sample importing should fail (depending on which version runs the import, it'll fail for a different reason).

Why not do this in 'cbimport'?
Although this logic could be handled by 'cbimport' I don't think it's the correct place for it to be handled; if we handled it, it would either 1) result in us returning a known error 2) result in the import "failing" silently (in both cases, the user has no sample bucket, and they aren't informed as to why not). I think the best place for this to be handled will be in 'ns_server' since users will be provided with feedback when they attempt to hit the endpoint to import a sample bucket e.g. 'BadRequest'