Changing the label for the Inbound Email activity type lets you edit/delete them even if the permission disallows

This is related to #1116 but I'd like to start with this one first since it's more contained and allows removing a confusing variable that is only ever used once and used incorrectly.

1. To reproduce, set the permission so that editing inbound emails is disallowed. Will probably already be like this by default.
2. Change the activity type label for Inbound Email.
3. Go to a contact's activities tab and click the view link for an inbound email. On the popup there's now edit and delete buttons whereas before there weren't.
4. Click the edit button, make your edit, and save.

The problem is this line in https://github.com/civicrm/civicrm-core/blob/5.15.1/templates/CRM/Activity/Form/Activity.tpl#L252

{if $action eq 4 && ($activityTName neq 'Inbound Email' || $allow_edit_inbound_emails == 1)}

$activityTName (which is different than $activityTypeName, although it has the same value) is actually the label, and so if you've changed the label the comparison doesn't work.

This is the only place this variable is used.