Subject summaries are loaded one at a time

[garethbowen]

In the GetSubjectSummaries service we loop through the subjectIds and issue promises to hydrate them one at a time. Instead we should batch the ids and fetch them all in a single call.

[garethbowen]

Real world performance improvements on my tecno Y4 with a small db:

Workflow	Before	After	Percent faster
Reload webapp on reports tab	28.0s	20.8s	135%
Report search (no filters)	12.5s	6.1s	205%
Contact search (no filters)	3.5s	3.2s	109%

[garethbowen]

Code review please @dianabarsan