Improvement/A11y: Audit SkipLink

[fpapado]

I went down a rabbit hole of browser bugs and mobile testing, after reading this https://axesslab.com/skip-links/.

Turns out, Skip Links might not get announced if tabindex="-1" does not exist on the focus target (i.e. #main in this case).
On the other hand, if tabindex=-1 is present, then the focus behaviour for tapping somewhere on the page can bring focus all the way to the top. (It really is not supposed to do that).

One potential solution is to only add tabindex right before the focus is moved, and remove it after. This relies on JS, but is largely a progressive enhancement. Without JS, the behaviour is still a bit "meh", because it seems to me that the tabindex bug can be more serious; or, another way, the announcement seems to be getting fixed.

Steps:

• Try out the solution by adding a js/SkipLink.js module
• Test on Android and iOS, both with and without (the PR app helps!)
• Decide on the way forward, document it
• Reach out to folks / write and share. I think it can help others!

Might want to look into various bug trackers from browsers.