Extracting html templates into a library (#16946)

* Extracting html templates into a library

* Moving tests to the right place

* Fixing tests

* Addressing PR review comments

* Addressing PR review comments

* Replacing attomic with RWMutex

* Returning errors as channel for Templates watcher

* Address PR review comments

* Other small fixes

* Simplifying NewWithWatcher

* Addressing PR review comments

* Making error handling on rendering templates more robust

* Fixing tests

* Changing how we return errors

* Fixing shadow variables

* Addressing PR review comments

* Logging errors from the outside of sendNotificationEmail

* Fixing lock in shutdown

* Fixing the resource copy for commands tests temporary directories

* Removing unused import

* A couple of tiny fixes

app/notification.go

@@ -231,7 +231,10 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 			}
 
 			if a.userAllowsEmail(profileMap[id], channelMemberNotifyPropsMap[id], post) {
-				a.sendNotificationEmail(notification, profileMap[id], team)
+				err := a.sendNotificationEmail(notification, profileMap[id], team)
+				if err != nil {
+					mlog.Warn("Unable to send notification email.", mlog.Err(err))
+				}
 			}
 		}
 	}