Ensure caches always check ETags on dynamic pages. Fixes #3547

@shane-tomlinson or @jrgm, mind having a look? I feel like this bug took me down a rabbit hole into an upside down world of eight-year-old blog posts/bugzilla bugs covering HTTP caching quirks in IE6 etc. Yuck!

Not sure the best way to test this. I considered actually changing the static endpoints and reloading the dialog, but it seems like there should be a way to do this via front-end unit test.