Consolidate MIME type configuration in securedrop-workstation-config package

[eloquence]

Currently, MIME types are provisioned via Salt and via packages (i.e. securedrop-client, securedrop-export and securedrop-workstation-svs-disp). As part of the template consolidation epic (freedomofpress/securedrop-workstation#471), we want to provide a set of MIME type configurations in the securedrop-workstation-config package, which would be installed in the TemplateVMs.

The package should contain:

• in /opt : all possible mimetype definitions for all AppVMs
• in /usr/share/applications: a “defensive” mimetype definition for to be installed in a template (default action for all mimetypes is to open in disposable VM)

This can be done initially in an additive way, without removing the existing MIME type handling logic.

(Issue drafted in collaboration with @emkll)