Fix/update watering status of tree cluster on sensor change

[doriengr]

Close #387

How to test it (frontend-wise):

• select a tree cluster where one linked tree in the tree cluster has a sensor. Check which sensor is linked
• select another tree cluster, select one random linked tree in the tree cluster and link the sensor to it that is currently linked to the tree in the first cluster and save your changes
• check if both watering status values are updated. the tree cluster without any sensors should have now »unknown« as a watering status value

[xk0niR]

How I tested:

• Create two treecluster tc1 and tc2

• Assign tree t1 to tc1, assign tree t2 to tc2

• Assign sensor s1 to t1
  => tc1 now shows "Sehr trocken" because of s1

• Assign sensor s1 to t2
  => tc1 now shows "Unbekannt" because no sensor left.
  => tc2 now shows "Sehr trocken" because of s1

• Assign sensor s1 to t1
  => tc1 now shows "Sehr trocken" because no sensor left.
  => tc2 now shows "Unbekannt" because of s1

Approved!