Copyedit and update "Browsers and devices"

- Minor style guide edits
- Rewrite first paragraph, remove unnecessary heading
- Remove all the links to GOV.UK
- Update statistics from StatCounter
- Link to Performance Platform GOV.UK dashboard
- Remove 'Understanding progressive enhancement' link (it's in the relevant guidance)
Update supported browser table

Using data from @edds' browser matrix, focusing specifically
on small screens:

- Add iOS 7 Google Chrome (it's approaching 1%)
- Add Android Browser 4.x (it's also around 1%)
- Add Windows Phone 8 IE 10 as functional (it's about 0.5%)
- Up BlackBerry requirement from 6 to 7.1, make it functional (0.2%)
Change browser tables to use rowspan attribute

To make them more accessible. Hat tip to @bradleywright.

https://github.com/alphagov/government-service-design-manual/pull/340#discussion_r11842934
Add TH and SCOPE where appropriate

This is required so DOM parsers can correctly infer which cells are
headings and which are content.

The TBODY in the table is implicit, which is why we only need to specify
THEAD wrappers.
Merge pull request #340 from alphagov/browsers-devices-update

Update "Browsers and devices"