This should replace native Android back behavior with either going
back in the webview history or closing the app.

In theory the react-router pushes history entries to the webview
navigation history, as soon as the user clicks a Link or navigate
is called. The navigate function can also be called with replace
option so that it does not add to the history.

This is using the native goBack method but it seems that the app
only goes back to account summary. In theory this should behave
the same as native browser back. It is not clear why it jumps to
the beginning.

So this could probably be improved, but seems already useful with
just going back to the account summary.

On the account summary it cannot go back any further so it will
trigger the old do-you-want-to-quit alert.

Replaced deprecated onBackPressed override.

The previous commit added a condition webview.canGoBack() and then
uses goBack() if the webview can go back in history.

This does not behave the same as native browser back in webdev.

The webview loads with loadDataWithBaseURL, where the 4th argument
is historyUrl which is null. Introduced in: 3b62772

This commit uses same base url to initialize historyUrl.

Alert component exposes a function to be called from anywhere.

This is a bit a hack and if called multiple times, last alerts
are lost.

Changed to check if alert is already active and append new messages
to the same alert.

seems sd there can be similar issues when using history back from
a webview that was loaded by loadDataWithBaseURL.

This is wild guess

https://stackoverflow.com/questions/3574674/android-webviews-method-goback-shows-a-blank-page

https://stackoverflow.com/questions/8974102/android-blank-screen-on-onback-in-a-loaddatawithbaseurl

trying recommendation from one of the answers from
https://stackoverflow.com/questions/8974102/android-blank-screen-on-onback-in-a-loaddatawithbaseurl?noredirect=1&lq=1

test to check if loadDataWithBaseURL with shiftcrypto.ch origin might
cause history to go back to "https://shiftcrypto.ch/"
introduced in 3b62772