Allow setting API token for dasboard exports in templates

Bumping this request. 

This is a very good feature and honestly could be a standalone selling point for the Better PDF exporter. Problem is that is uses a particular users API to complete the export. This means that every user who wants to export a dashboard needs to generate and use their own API key. 

Not only is this more work for end users, but also for larger companies who lock down creation of API keys to only Administers, it creates a lot of friction with having users share Dashboards, and the queries they rely on, to administrators to be able to export dashboards to PDFs. 

This would be much improved if similarly to the normal Export Issue to PDF, if administrators were able to hard code a API into the export template that would allow all users to export their own dashboards without creating API tokens, or having to get Administrators involved.