Analytics: Make fields sortable

[winged]

Analytics fields should be sortable. Analog to form questions, a ReorderAnalyticsFields mutation should be implemented. On DB level, the fields should receive a sort attribute that's updated according to the requested ordering. Additionally, new fields should get added to the end by calculating the "next" sorting value.

On the output side, the columns of the analytics (generated in caluma_analytics/schema.py, in AnalyticsOutput, should be sorted accordingly as well.

[open-dynaMIX]

/cc @Yelinz