Add advice on credential strength to logins page

Cherry-pick 1ea0f1d which failed due to submodule jiggery-pokery.
Add note about ssl

Cherry-pick of 4f840bbfc795fb39f326e15bec2a519219165b4a
More consistent use of passphrase vs. password

Cherry-pick of dc7963c725f00e6f5a52d02f1bad4a78d3fa1918.
XKCD 936 link

Cause you can't talk about passphrases without it.

Cherry-pick of fefea16374c1e5945d2ef81220075c46c36d1606.
Capitalise HTTPS
The sound of the police
Format for hopefully easier diffs
Style guide – use lists
Prefer simpler word
Style guide – prefer contractions in most cases
Whitespace for hopefully cleaner diff
Merge pull request #224 from alphagov/login-guidance

Login guidance