improving perf of profile photos

per 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. This work will be done in APA - 155
Optional: Consider sending a GET with a non-empty body to avoid possible url length limit issues, assuming the client and server are okay with that.  (e.g. https://github.com/socialcast/elastic_searchable/blob/166294976fd91dd07a9f3283d31bcb3f54e134bd/lib/elastic_searchable/active_record_extensions.rb#L139 )