Add IE9-specific stylesheet
Remove fancy select styling for IE <= 9 on stats announcement filter

The select dropdown arrows can't be hidden without some serious hackery
in IE9 and can't be hidden at all in IE <= 8. It's better to strip back
any fancy styling for IE.
Use same styling for publication & stats announcement filters

Previously, the styling for the publication filter and stats announcement
filter were using two different versions of similar css. This modifies the
publications filter to use the more generic filter-form-block css helper.
Remove logos from 'exempt' organisations

We don't want to have to maintain logos for exempt organisations, so we're
not going to show them.
Fix custom logo crest option label on organisation edit page

Organisations can be 'exempt' in two ways:
• Exemption from using the Single Identity branding
• Exemption from having to publish on GOV.UK

The label for the select option to use a Custom Logo was ambiguous as
to which of these exemptions it was referring to.
Validate that exempt organisations do not have custom logos
Merge pull request #1581 from alphagov/remove-logo-from-offsite-org-pages

Remove logo from offsite org pages
Merge pull request #1577 from alphagov/fix-selects-in-ie

Fix selects in ie

Story: https://www.pivotaltracker.com/story/show/73412800

Previously, select boxes on the frontend filters were displaying with two down arrows - one from the background image, and one browser-rendered dropdown arrow. This PR strips out the more complicated select styling for IE version <= 9.

I needed to introduce an IE9 stylesheet to achieve this.

This PR also rationalises the styling between the publications filter and the stats announcement filter.
Add ability for LinkChecker to check authed links

Because the preview environment is hidden behind HTTP basic auth, link
checking will fail for any links to other pages on the site in this
environment. This change allows us to configure domains that require
authentication such that the LinkChecker will be able to automatically
apply the auth credentials when required. This will be configured at
deploy time so that environment-specific details are kept out of the
codebase.
Merge pull request #1586 from alphagov/allow-authed-domains-to-be-link-checked

Add ability for LinkChecker to check authed links