This uses pytest's "markers" system to add markers to two special types
of tests:

* webtest - ones that use the WebTest library and are testing the actual
  HTTP app, instead of executing code/functions directly
* html_validation - ones that are generating HTML output (via webtest)
  and running it through the Nu HTML Checker to validate it.

The "webtest" marker is added automatically by checking whether a test
uses either of the webtest fixtures, and the html_validation one is
currently added manually to the only module that has those tests. In the
future, we could probably put HTML validation tests in their own folder
and mark them automatically based on the module's path or something
similar.

This also changes the default arguments for pytest to exclude these two
marked types of tests, and updates the git hooks so that webtests are
run pre-commit (but not HTML validation), and all tests are run
pre-push. Similar to the way we use prospector, this makes it so that
the very slow tests are only run before pushing.