fix(subscriptions): cleanup and simplify fetching subscribers for tasks and projects logic

Vikunja now uses one recursive CTE and a few optimizations to fetch all subscribers for a task or project. This makes the relevant code easier to maintain and more performant.

(cherry picked from commit 4ff8815fe1bfe72e02c10f6a6877c93a630f36a4)

pkg/utils/strings.go

@@ -0,0 +1,31 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public Licensee as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public Licensee for more details.
+//
+// You should have received a copy of the GNU Affero General Public Licensee
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package utils
+
+import "strconv"
+
+func JoinInt64Slice(ints []int64, delim string) string {
+	b := ""
+	for _, v := range ints {
+		if len(b) > 0 {
+			b += delim
+		}
+		b += strconv.FormatInt(v, 10)
+	}
+
+	return b
+}