MM-27184 deprecate model.SetExpireInDays (#15165)

Mobile users were having their sessions unexpectedly expired, despite having ServiceSettings.ExtendSessionLengthWithActivity enabled. 

Every time a mobile app is opened it called `/api/v4/sessions/device` which calls attachDeviceId which calls `(*Session)SetExpireInDays`. This code above assumed the expiry should be relative to CreateAt which is incorrect when ExtendSessionLengthWithActivity is enabled. Therefore, every time the mobile app was opened, the maximum expiry was set in memory to CreateAt + session_length, even if the session was extended.

(*Session)SetExpireInDays is now deprecated and replaced with (*App)SetSessionExpireInDays which takes into account the ExtendSessionLengthWithActivity setting.

app/session_test.go

@@ -350,3 +350,55 @@ func TestApp_ExtendExpiryIfNeeded(t *testing.T) {
 	}
 
 }
+
+const (
+	dayInMillis = 86400000
+	grace       = 5 * 1000
+	thirtyDays  = dayInMillis * 30
+)
+
+func TestApp_SetSessionExpireInDays(t *testing.T) {
+	th := Setup(t)
+	defer th.TearDown()
+
+	now := model.GetMillis()
+	createAt := now - (dayInMillis * 20)
+
+	tests := []struct {
+		name   string
+		extend bool
+		create bool
+		days   int
+		want   int64
+	}{
+		{name: "zero days, extend", extend: true, create: true, days: 0, want: now},
+		{name: "zero days, extend", extend: true, create: false, days: 0, want: now},
+		{name: "zero days, no extend", extend: false, create: true, days: 0, want: createAt},
+		{name: "zero days, no extend", extend: false, create: false, days: 0, want: now},
+		{name: "thirty days, extend", extend: true, create: true, days: 30, want: now + thirtyDays},
+		{name: "thirty days, extend", extend: true, create: false, days: 30, want: now + thirtyDays},
+		{name: "thirty days, no extend", extend: false, create: true, days: 30, want: createAt + thirtyDays},
+		{name: "thirty days, no extend", extend: false, create: false, days: 30, want: now + thirtyDays},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			th.App.UpdateConfig(func(cfg *model.Config) {
+				*cfg.ServiceSettings.ExtendSessionLengthWithActivity = tt.extend
+			})
+			var create int64
+			if tt.create {
+				create = createAt
+			}
+
+			session := &model.Session{
+				CreateAt:  create,
+				ExpiresAt: model.GetMillis() + dayInMillis,
+			}
+			th.App.SetSessionExpireInDays(session, tt.days)
+
+			// must be within 5 seconds of expected time.
+			require.GreaterOrEqual(t, session.ExpiresAt, tt.want-grace)
+			require.LessOrEqual(t, session.ExpiresAt, tt.want+grace)
+		})
+	}
+}