Calculate the sponsored percentage for a Site

[NicolasDontigny]

Currently, the % sponsored number is just a random number returned from the back-end. We need to implement the actual calculation for this number. Here is the calculation:

[Sum of all total number of trees for all batches of the site] / [Total number of trees in the site]

Story #242 Makes sure that the sum of the total number of trees in the batches cannot exceed the total number available in the site, although it is not necessarily a requirement for this story, both can be done independently