Improve performance of queries filtering by single enttiy

[muety]

1. wakapi/services/summary.go

   Line 105 in 85515d6

   if filters == nil || filters.IsEmpty() {

   -> for single-entity filters, we can just fetch the summary

2. wakapi/services/summary.go

   Line 136 in 85515d6

   durations, err := srv.durationService.Get(from, to, user, filters)

   -> for single-entity filters, we can let the database handle filtering

Will especially improve performance of the badges endpoint, as not all heartbeats of that user within that interval have to be fetched.