Fix intermittent crash

If a status in a conversation view controller creates a work item to
update the timestamp in 1 minute, but the view controller is deinit'd
before that time elapses, the mastodonController instance will be nil,
resulting in a crash.

The DispatchWorkItems's are cancelled by the respective cell deinit
methods. But if the work item has already begun, cancelling it has no
effect, potentially leading to a crash in the conditions described above
are true. Using a weak reference to self fixes this.

Additionally, don't unnecessarily recreate the work items every time.
They don't capture any local variables, only self, so nothing changes.

Tusker/Views/Notifications/ActionNotificationGroupTableViewCell.swift

@@ -115,8 +115,10 @@ class ActionNotificationGroupTableViewCell: UITableViewCell {
             delay = nil
         }
         if let delay = delay {
-            updateTimestampWorkItem = DispatchWorkItem { [unowned self] in
-                self.updateTimestamp()
+            if updateTimestampWorkItem == nil {
+                updateTimestampWorkItem = DispatchWorkItem { [weak self] in
+                    self?.updateTimestamp()
+                }
             }
             DispatchQueue.main.asyncAfter(deadline: .now() + delay, execute: updateTimestampWorkItem!)
         } else {

Tusker/Views/Notifications/FollowNotificationGroupTableViewCell.swift

@@ -104,8 +104,10 @@ class FollowNotificationGroupTableViewCell: UITableViewCell {
             delay = nil
         }
         if let delay = delay {
-            updateTimestampWorkItem = DispatchWorkItem { [unowned self] in
-                self.updateTimestamp()
+            if updateTimestampWorkItem == nil {
+                updateTimestampWorkItem = DispatchWorkItem { [weak self] in
+                    self?.updateTimestamp()
+                }
             }
             DispatchQueue.main.asyncAfter(deadline: .now() + delay, execute: updateTimestampWorkItem!)
         } else {

Tusker/Views/Notifications/FollowRequestNotificationTableViewCell.swift

@@ -78,8 +78,10 @@ class FollowRequestNotificationTableViewCell: UITableViewCell {
             delay = nil
         }
         if let delay = delay {
-            updateTimestampWorkItem = DispatchWorkItem { [unowned self] in
-                self.updateTimestamp()
+            if updateTimestampWorkItem == nil {
+                updateTimestampWorkItem = DispatchWorkItem { [weak self] in
+                    self?.updateTimestamp()
+                }
             }
             DispatchQueue.main.asyncAfter(deadline: .now() + delay, execute: updateTimestampWorkItem!)
         } else {

Tusker/Views/Status/TimelineStatusTableViewCell.swift

@@ -110,8 +110,10 @@ class TimelineStatusTableViewCell: BaseStatusTableViewCell {
             delay = nil
         }
         if let delay = delay {
-            updateTimestampWorkItem = DispatchWorkItem { [unowned self] in
-                self.updateTimestamp()
+            if updateTimestampWorkItem == nil {
+                updateTimestampWorkItem = DispatchWorkItem { [weak self] in
+                    self?.updateTimestamp()
+                }
             }
             DispatchQueue.main.asyncAfter(deadline: .now() + delay, execute: updateTimestampWorkItem!)
         } else {