[#847] Clarify about variables passed into snippets
[#847] Add a TODO
[#847] Change dataset of the day example to most popular groups

Change the dataset of the day example in the theming docs to a most
popular groups example.
[#847] Update a Sphinx reference
[#847] Add limit param to recently_changed_packages_activity_list template helper function
[#847] Add an HTML and CSS example to the theming tutorial

Also remove a duplicated section about global template variables.