Add a "grid imports" limit

[brtietz]

Is your feature request related to a problem? Please describe.

The grid export limit is a really useful feature for interconnection limits, but it does not affect grid imports for battery charging. There are situations where export versus import can be imbalanced (especially for distributed systems) so a "grid import limit" would be a useful feature.

Describe the solution you'd like

Add a new input to the grid limits page for grid imports.

Describe alternatives you've considered

Battery size can limit this, but in some cases the battery may need a larger power to charge from the grid and the generator simultaneously.

Additional context

While changing limits on the grid limits page, consider #1388 at the same time