Re-write of learning disabilities paragraph

Make the tone a bit more formal in keeping with the rest of the document.
Removed superfluous final sentence which is covered in next paragraph titled "Why we do this"
Update how-users-read.md

Clarified plain English section, tidied up grammar and fixed style points.
Fix repeated "not"

From "the standard itself will not not apply" to "the standard itself will not apply".
update blue hex values

to match https://github.com/alphagov/govuk_frontend_toolkit/pull/80
Update toolkit to latest version

Gets this lot:
https://github.com/alphagov/govuk_frontend_toolkit/compare/ccbc4057...1c4f5a26

Highlights: new blue, some more images, more javascript.
Merge pull request #304 from ohskylab/patch-1

Fix repeated "not" typo
Add the media player back

The media player was broken with the bump to govuk_template. This brings
it back. It now uses the stylesheet from the template. jquery ui is
needed to let the slider work. This is the minium jquery ui needed to
make it work.
Merge pull request #305 from alphagov/video-player

Add the media player back
Merge pull request #297 from alphagov/blues-become-one

update blue hex values
Merge pull request #292 from alphagov/copy-change-how-users-read

Re-write of learning disabilities paragraph