Inconsistent email header option naming

[tomas-goncalves]

Currently when you load a page with an email there is an option to "View All Headers". Clicking this changes the text to "Hide Extra Headers". Clicking the toggle again changes the text to "View Hidden Headers". The name should probably be the same all the times (maybe extra headers as it is independent of the current rendering state). I can PR a fix later today if "Extra Headers" sounds ok.

[NoRePercussions]

Sounds good. You could consider just "Show Headers" and "Hide Headers" as well; it is not perfectly accurate but it is descriptive. I'm not picky about what term you settle on.