Show inhibited alerts

[geisslet]

Add a possibility to make inhibited alerts visible.

Not sure if this should be just added to the main view by filter flag, because this would be similar to alertmanager itself causing less transparency.

Could be also be a separat view like requested for maintenance silences, because you do that as a specific use case (or it's a new major filter selectable on the left navigation bar (landscape, support ..)

@artherd42