FIX 'Activities Balance' is shown as negative on advocate page

[tyagishuchi1]

The activities available balance is not calculated properly and is shown as a negative value.

Expected Behavior

Activities Available => It should be calculated for the logged in user. It is the sum of all the unassigned slots in the ‘Available activities’. For example, for the set of activities given below it should be 275

Current Behavior

Steps to Reproduce

Environment: Temp

1. Assign a slot
2. Mark it complete to initiate a reward transfer