activity: add filtered count route

separated the search parts of ServerSearchReqDTO into
ServerFilterReqDTO, factored out part of GetActivities that made the
(filtering) query, which is then used in GetFilteredActivityCount with
db.Count.

usercache.go

@@ -499,6 +499,11 @@ func (q *QueryDTO) UnmarshalJSON(data []byte) error {
 // ServerSearchReqDTO is a usual PaginatedReqDTO with added fields for searching and filtering.
 type ServerSearchReqDTO struct {
 	PaginatedReqDTO
+	ServerFilterReqDTO
+}
+
+// ServerFilterReqDTO provides search terms and queries to a search or count route.
+type ServerFilterReqDTO struct {
 	SearchTerms []string   `json:"searchTerms"`
 	Queries     []QueryDTO `json:"queries"`
 }