Fix Issues 806, 813, 815, 816, 817, 818

815: Corrected indentation for {% if completed_missions %}
816: Changed the mission status on the main page (for logged in users) to use the same CSS class as the sub-mission progress classes on the individual missions page.
813: Fixed a grammar error in the "High" level hint on the recursive diff mission page.
818: Added a curl alternative for mac users. Changed it from tar zxvf to tar -xvf and added a gzip -d command, to make it mac compatible.
806: Almost same fix as 818, but make curl the main option. Improve explanation of gzip -d
817: Add suggestion to look at the Medium hint, in the error message