Add additional metadata to post reminder (#22306)

The webapp needs to construct the full message because
the team_name needs to be inserted into the permalink.

So we need to send everything in the metadata for the
client to construct the message.

Also, we set a custom post type for the client to easily
add a custom condition.

```release-note
NONE
```

model/post.go

@@ -415,6 +415,7 @@ func (o *Post) IsValid(maxPostSize int) *AppError {
 		PostTypeAddBotTeamsChannels,
 		PostTypeSystemWarnMetricStatus,
 		PostTypeWelcomePost,
+		PostTypeReminder,
 		PostTypeMe:
 	default:
 		if !strings.HasPrefix(o.Type, PostCustomTypePrefix) {