MM-16258: Adds new API endpoint + (App & Client & Store) to retrieve … (#11176)

* MM-16258: Adds new API endpoint + (App & Client & Store) to retrieve Users who would be removed from a list of hypothetical group IDs representing the synced groups.

* MM-16258: Adds roles to JSON response.

* MM-16258: Updates GetByIDs to use Squirrel.

* MM-16258: Puts as much as possible into Squirrel.

* MM-16258: Changes names of methods, functions, and route.

* MM-16258: Updates some comments.

* MM-16258: Extra validation of group_ids parameter.

* MM-16258: Changes validation of group_ids query param.

* MM-16258: Rename a variable and a constant.

* MM-16258: Fix test.

model/user.go

@@ -8,6 +8,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"net/http"
 	"regexp"
 	"sort"
@@ -768,3 +769,24 @@ func IsValidLocale(locale string) bool {
 
 	return true
 }
+
+type UserWithGroups struct {
+	User
+	GroupIDs    string   `json:"-"`
+	Groups      []*Group `json:"groups"`
+	SchemeGuest bool     `json:"scheme_guest"`
+	SchemeUser  bool     `json:"scheme_user"`
+	SchemeAdmin bool     `json:"scheme_admin"`
+}
+
+type UsersWithGroupsAndCount struct {
+	Users []*UserWithGroups `json:"users"`
+	Count int64             `json:"total_count"`
+}
+
+func UsersWithGroupsAndCountFromJson(data io.Reader) *UsersWithGroupsAndCount {
+	uwg := &UsersWithGroupsAndCount{}
+	bodyBytes, _ := ioutil.ReadAll(data)
+	json.Unmarshal(bodyBytes, uwg)
+	return uwg
+}