Remove all page fixtures

[jf-x-dev]

This PR changes how page objects are used in tests.

Currently we have a fixture for every individual page object, and in each test these fixtures are passed as arguments. This is how we use the functionality of each of these page objects.

There are some problems with this approach though:

• There isn't actually any reason for these fixtures to exist
• Tests that access many pages end up having a very large number of arguments
• Adding new page objects requires adding new fixtures etc. which ends up requires changes in multiple files

A simpler approach is to just pass the Playwright page fixture to tests, and then create the necessary page objects in the test (e.g. calling LogInPage(page).log_in()). This allows us to:

• Remove all page fixtures (mavis/test/fixtures/pages.py deleted)
• Massively reduce arguments required for each test, making them easier to understand and edit
• Make adding page objects much easier

[thomasleese]

Makes sense to me, I've also noticed that having so many of these fixtures is a bit of a pain to work with!