Add filter by maximum power

[johan12345]

For this we could turn the power filter into a RangeSlider.

OpenChargeMap already has this filter on the server side (maxpowerkw). For GoingElectric we would need to implement it on the client side.

Logic should be: If maximum power filter is set to x kW, only locations where the minimum power of any connector at the location is below x kW will be shown (i.e. it is allowed to have higher power connectors as well, but at least one must be below the maximum).
When combined with the connector filter, this only applies to connectors with the chosen socket(s). When combined with the "minimum number of connectors" filter, there must be at least n connectors with power below the maximum.

Requested in GoingElectric forum.

Use case: For certain charging providers, it may be cheaper to charge at stations with lower power.