Add PWA manifest enabling installation on android

[hexylena]

This PR adds a 'manifest.json' which enables PWA features, and mainly that the user has the ability to "Install" the application.

The "installation" works best on android when done via chrome, as firefox does the simpler "shortcut" method even for proper PWAs, Chrome will send the site to their server which turns it into a shim APK which will then show up in the user's app list. Which makes it easier to discover and nicer for users.

Additionally a logo is added as that is required, I opted for the simple "W from wikipedia poorly censored" but please replace it if you like, it took 60 seconds to make and I needed the placeholder :)

(Here a screenshot from an earlier draft where I used a shorter version of the name for short_name)

Additional changes includes:

• A service worker for the PWA. This is described as mandatory for PWAs but I have evidence to the contrary. Nevertheless it is included as it also provides a completely transparent cache for requests going to wikipedia which is nice for us and them.
• improving metadata which results in nicer link previews when shared on social media (again please feel free to update this description accordingly)
• yeah that's pretty much it

[larsgw]

There's probably no way to adapt the theme color to the user settings right? There's https://github.com/whatwg/html/pull/6569 (which is cool) but that is for the system settings.

[hexylena]

I don't think so, without a dynamic server component. I've seen PWAs use dark and light colors for the theme_color field, and generally they match the header/nav colour.

[larsgw]

Sorry for the delays? What do you think of this as a logo?

[hexylena]

Oh, that's way better! Love it!