Refactor question list

There's no longer a `list-questions` command. Instead, we have a `sx-tab-frontpage` command.

There's also a `sx-tab--define` function, which will greatly simplify defining new tabs.
It defines a command which creates a buffer in `sx-question-list-mode` and sets up to 3 variables necessary for the mode to function.
To know how to use these variables, read their docs and the doc of `sx-question-list-mode`.

I'll try to update sx.org later this week. If I have time.
Good call.