Show a notification to install the Sponsorkliks extension

[wilco375]

We can now detect if the Sponsorkliks extension is installed because it adds the data-sponsorkliks-extension="1" property to the html tag of the website. So it would be nice to add a notification to the website somewhere on the homepage for logged in users that asks them to install the Sponsorkliks extension if it is not installed yet. Because it is basically free money for Alpha 😃

[Matthijsy]

Already in PR #153 :) Would be nice to get a review

[wilco375]

Ah nice, we had the same thought