Activate AppVM MIME type configuration via securedrop-workstation-config package

[eloquence]

As part of template consolidation (#471), once freedomofpress/securedrop-builder#188 is resolved, we can transition to using the securedrop-workstation-config package to configure MIME types in all AppVMs. In order to do so, we will need to symlink the correct MIME type configuration (provided by the securedrop-workstation-config package) for a given AppVM into its private volume, e.g. to /home/user/.local/share/applications/mimeapps.list.

The MIME database is updated from this source file via the update-mime-database command. We need to ensure that the database is updated whenever required, and have so far discussed two implementation options:

• at boot time via /rw/config/rc.local, and/or
• during provisioning

The initial work here is to complete a research spike to better understand which implementation strategy is viable (e.g., in a VM like sd-proxy).

Acceptance Criteria

To fully resolve this issue:

• MIME type configurations for all Workstation AppVMs are selected from the configurations included in the securedrop-workstation-config package
• Now redundant MIME type configurations provisioned via Salt or Debian packages have been removed.

[eloquence]

Possibly related: #567 discusses implementation of a generic mechanism for enforcing Salt states beyond dom0, which could come in handy if we don't want to rely on boot time updates. Basically, whenever we update the MIME configuration, we'd have to ship an RPM update that instructs the updater to enforce the Salt state for the impacted AppVMs.