[MM-24453] Sanitize user term search (#14494)

* Sanitize user term search

Remove the @ at the beginning of the search term in order to
make it possible for ES to return results

store/searchlayer/utils.go

@@ -0,0 +1,10 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package searchlayer
+
+import "strings"
+
+func sanitizeSearchTerm(term string) string {
+	return strings.TrimLeft(term, "@")
+}