Further improve WebView styling

[Fs00]

This PR makes more improvements to WebView styles, with a few more fixes and tweaks:

• fix printing of Markdown previews, which was not working anymore after leveraging JavascriptInterfaces more extensively
• change background color of WebViews to be exactly the same as the app UI, by setting it to transparent.
  Previously it was set to a slightly different shade of grey compared to the other screens.
• improve CSS styles, particularly spacing and text contrast in dark theme as you can see below:

Before	After

• markdown CSS styling is now also applied to wiki pages and GH blog posts. It makes them look considerably better, as images do not overflow the page anymore:

Before	After

• images will now fit the screen size in file viewer (previously they were zoomed in if bigger than the screen)
• extra: disabled filesystem access in WebViews for improved security. It is never used by the app as far as I've seen.