Hackathon: Post Reminders (#20555)

This PR adds the post reminder backend work.

We add a new API endpoint via which a user can set a reminder for a post. An ephemeral message will be sent down the line to let the user know about the action. And then after the time is over, the system admin bot will send a DM message to the user about the reminder post.

model/post.go

@@ -48,6 +48,7 @@ const (
 	PostTypeSystemWarnMetricStatus = "warn_metric_status"
 	PostTypeMe                     = "me"
 	PostCustomTypePrefix           = "custom_"
+	PostTypeReminder               = "reminder"
 
 	PostFileidsMaxRunes   = 300
 	PostFilenamesMaxRunes = 4000
@@ -149,6 +150,13 @@ type PostPatch struct {
 	HasReactions *bool            `json:"has_reactions"`
 }
 
+type PostReminder struct {
+	TargetTime int64 `json:"target_time"`
+	// These fields are only used internally for interacting with DB.
+	PostId string `json:",omitempty"`
+	UserId string `json:",omitempty"`
+}
+
 type SearchParameter struct {
 	Terms                  *string `json:"terms"`
 	IsOrSearch             *bool   `json:"is_or_search"`