Install a backport of shared-mime-info 1.2+

Originally created by @intrigeri on #7039 (Redmine)

See the parent ticket for the rationale.

Feature Branch: feature/6608-OpenPGP-signature-verification-in-Nautilus

Edited May 15, 2020 by intrigeri

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information