Allow filtering by resource level

[kosayoda]

Requires #449.
Once the resources are properly tagged, it would be good to have a filtering system for experience level.

This can be done with a dropdown menu next to each section in each resource.

Here's one such mock implementation:

Anyone who wants to implement this can propose alternative implementations, feel free to do so in #dev-contrib or #dev-dewikification.

[lemonsaurus]

I'm closing this in favor of #496, to help prevent confusion. This will be solved by that issue.