Make welcome content more readable

[veyndan]

Required for #1013.

The welcome content can be quite unreadable across certain screen sizes.

1. Firefox for Linux mobile implies that <meta name="viewport" content="width=device-width, initial-scale=1.0"> is already present. This makes the lines in the content very short, which is annoying to read.
2. Firefox for Android doesn't imply that <meta name="viewport" content="width=device-width, initial-scale=1.0"> is already present. This makes the content appear to be very zoomed out, making the actual text appear very small and hard to read.
3. Firefox for desktop on a widescreen monitor makes the text span a lot of the monitor. Since it's recommended by the WCAG to not exceed 80 characters per line for accessibility purposes, I've clamped the line width. Note that I didn't ensure that we're actually clamping up to 80 characters and no more due to the way ch works (our best bet of achieving this). In order to get about 80 characters per line, I set the width to 60ch, as recommended by Eric Meyers.

Note that, since this extension is only installable on Firefox desktop and Firefox on a Linux phone, the Firefox on Android "screenshot" was created via the Responsive Design Mode in Firefox Dev Tools. It is, however, very representative of what it'd look like on Android.

Device	Before	After
Firefox on Linux mobile
Firefox on Android (theoretically)
Widescreen monitor on Firefox desktop