[MM-37933] Channel preference to auto-follow all threads in the channel (#21430)

* Add auto-follow feature
---------

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/model/utils.go

@@ -41,8 +41,26 @@ const (
 var ErrMaxPropSizeExceeded = fmt.Errorf("max prop size of %d exceeded", maxPropSizeBytes)
 
 type StringInterface map[string]any
+type StringSet map[string]struct{}
 type StringArray []string
 
+func (ss StringSet) Has(val string) bool {
+	_, ok := ss[val]
+	return ok
+}
+
+func (ss StringSet) Add(val string) {
+	ss[val] = struct{}{}
+}
+
+func (ss StringSet) Val() []string {
+	keys := make([]string, 0, len(ss))
+	for k := range ss {
+		keys = append(keys, k)
+	}
+	return keys
+}
+
 func (sa StringArray) Remove(input string) StringArray {
 	for index := range sa {
 		if sa[index] == input {