Possibility to reorder tutorials

[grst]

Some tutorials are particularly recommended to get started. They should appear as the first tutorial(s) in each listing.

Proposed solution:
Add an optional "sort" key to the schema that can be filled with an arbitrary integer and sort by that. Entries without a "sort" key will have lowest priority.

[Zethson]

@VladimirShitov might be interested in tackling this one