ui#833 - Open web-based first-run page

This MR is for https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/-/issues/833

The first commit restores the first-run page's warning section which appears to have been removed by accident as part of https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/-/issues/589 because we ended up keeping it around for Firefox Mobile. It also escpaes [0]: support@adblockplus.org because it caused the following instance of [0] in the warning section to turn into a hyperlink.