improving perf of profile photos

per https://jira-projects.socialcast.com/browse/APA-148

We should batch requests users from Socialcast and do such outside the LDAP connection. 

With 2,000 users locally I saw these perf improvements:
```
# Request per user
real	6m58.543s
user	0m14.652s
sys	0m2.588s
 
# Batch request @ 50 per request
real	1m3.021s
user	0m3.686s
sys	0m0.605s
```

As suggested by @phene we could take this one step further and use https://github.com/grosser/parallel to thread the requests. If we deem that necessary I will make another ticket to do such