Add filtering for custom URI schemata (e.g. `mailto:`, `skype:`) to the filterClickEvent to force these to open outside the sandbox

[Jaifroid]

It turns out that attempting to open a mailto: link in a Chromium extension (or in a PWA in Chromium browsers) is seen by Chrome as an insecure protocol, which triggers sandbox blocking. See https://github.com/kiwix/kiwix-tools/issues/680. In Zimit1, the ReplayWorker handled this kind of stuff, I think, but we may have to handle it ourselves. There's currently already such a filter in JQuery/Safe mode.

However, we should first try a simple fix by adding mailto: as an allowed protocol in the sandbox Content Security Policy.