Loss factors widget for batteries

[brtietz]

Add a loss factors widget to the battery losses field.

Add an availability loss widget to the battery. This should limit both power and capacity of the battery.

AC connected batteries should not be affected by PV losses. In the DC connected case, the battery availability loss affects the DC output power, and the AC loss affects discharging or charging from the grid as it does now (test this to make sure it makes sense).

See #985 for more details