MP6ify the install / update screens

[johnbillion]

I'll take a look at this.

[johnbillion]

In 25951.diff​:

This patch affects the following screens:

• Creating a wp-config.php file
• Installation
• Database upgrade
• Database repair
• Readme.html
• Network sites actions (oddly enough)

What's changed:

• Bring background/text/link/border colours in line with mp6
• Bring fonts in line with mp6 (see note below)
• Switch to device-width for the viewport and tweak margins etc so the layout works nicely on all device sizes
• Switch to the new vector WordPress logo in the header to match the log in screen
• Force table cells into rows in the <782px media query

Notes:

• Open Sans is specified as the font face, but I've not added the font definition to the header. See #26072.
• The installation error screens use wp_die() (eg. if we can't connect to the database) which is styled separately. See #25956

[iammattthomas]

In 26298:

Bring the install/update screens in line with the new admin design. This affects:

• Creating a wp-config.php file
• Installation
• Database upgrade
• Database repair
• Readme.html
• Network sites actions

What's changed:

• Bring background/text/link/border colours in line with mp6
• Bring fonts in line with mp6 (see note below)
• Switch to device-width for the viewport and tweak margins etc so the layout works nicely on all device sizes
• Switch to the new vector WordPress logo in the header to match the log in screen
• Force table cells into rows in the <782px media query

Fixes #25951, props johnbillion.