End to end testing of various posts, sites, and media

[jmafoster1]

Issue 136

Additional playwright end to end tests for the links from the test document. I'm afraid I haven't been with the project long enough to know correct behaviour, so this will need checking. All the tests have passed for me, but can be a little flakey (e.g. if one of the third party services times out or fails). I think it would be good to try to make this a github action, but only if we can make it less flakey!