Extract message cache into message/ package with model/ types

Move message cache from server/message_cache.go into a dedicated
message/ package with Store interface, SQLite and PostgreSQL
implementations. Extract shared types into model/ package.

server/server_twilio.go

@@ -11,6 +11,7 @@ import (
 	"text/template"
 
 	"heckel.io/ntfy/v2/log"
+	"heckel.io/ntfy/v2/model"
 	"heckel.io/ntfy/v2/user"
 	"heckel.io/ntfy/v2/util"
 )
@@ -76,7 +77,7 @@ func (s *Server) convertPhoneNumber(u *user.User, phoneNumber string) (string, *
 
 // callPhone calls the Twilio API to make a phone call to the given phone number, using the given message.
 // Failures will be logged, but not returned to the caller.
-func (s *Server) callPhone(v *visitor, r *http.Request, m *message, to string) {
+func (s *Server) callPhone(v *visitor, r *http.Request, m *model.Message, to string) {
 	u, sender := v.User(), m.Sender.String()
 	if u != nil {
 		sender = u.Name