Improve feedback for Edge users who cleared cookies

Background

Microsoft Edge browser has problems with storage.local performance, to the point where Adblock Plus was nearly unusable. As a workaround we're using localForage (soon IndexedDB directly #6621) since the performance is much better. The disadvantage is that if the user clears cookies and other browser data IndexedDB / localForage are cleared too. That results in the user repeatedly seeing the first run page and otherwise loosing their settings.

Worse still since we started showing a warning to users with corrupted storage on other platforms (#6599) it has started showing up for some of the Edge users who cleared their browser data. It would be much better if we could display a different message to those users instead, especially important is that we don't tell them to try reinstalling the extension!

What to change

• Modify the firstRun_dataCorruptedWarning_* messages to explain to Edge users that their storage has been lost since they have cleared their browsing data.
• Push those changes to a separate edge branch of adblockplusui which is based upon the current revision of adblockplusui as listed in adblockpluschrome/dependencies under the edge branch of adblockpluschrome.

Hints for testers

Test that when the user clears their browsing data in Edge that a more useful message is displayed on the first run page, instead of the standard "Your data has been corrupted" one.