Load Previous for Admins doesn't load messages

Problem/Motivation

When viewing a thread between 2 users as an administrator, the load previous button doesn't load previous messages.

Steps to reproduce

1. Create a chat between 2 users A and B that have more than 25 messages so the load previousbutton appears.
2. View the thread as an administrator C
3. Click the load previous button.
4. The load previous button disappears, but no messages are loaded

Proposed resolution

The ajaxCallback currently only checks $this->currentUser->hasPermission('use private messaging system') and runs the following

 case 'get_old_messages':
          $this->getOldPrivateMessages($response);

This eventually calls getPreviousMessages() function and checks if the current user is in the thread to load more. Instead, we should have a check for $account->hasPermission('administer private messages') and load older messages.

I think using the checkAccess() function in the PrivateMessageThreadAccessControlHandler.php for the ajaxCallback would be best, then we can remove the $private_message_thread->isMember($this->currentUser->id()) check inside the getPreviousMessages() function.

Alternatively, check permissions in the ajaxCallback and call a new/different function in the get_old_messages which doesn't check permissions for users who have the 'administer private messages' permission.

Remaining tasks

Implement

User interface changes

None

API changes

None

Data model changes

None