Only show actions if there is actually an exporter for it.

Draft for now because of an issue that prevents PrintableHtmlDocument to be moved to icingaweb Icinga/icingaweb2#5510

• Remove PDF download option and mail attachment type when there is no pdfexport hook implementation registered
• Remove download dropdown and send button entirely if no export option is available

This serves as a step toward getting rid of pdfexport as a requirement for reporting, but does not get rid of this dependency entirely.

This combined with the changes made in #275 should allow us to remove pdfexport as a requirement. It is still not possible to use any module that implements the Pdfexport hook but at least the module is able to show reports in the web app and export to CSV and JSON.

• requires Add a hasLinks method to Dropdown ipl-web#367
• requires Remove dependency on pdfexport module for PDF export icingaweb2#5510
• relates to Switch back to using syncronous pdf export #275