Fix cookie settings page crash in IE11

[AshGDS]

What

• The cookie settings code has a line to grab the document referrer. It partly used the URL object for this.
• URL is unsupported in IE11, so it was crashing on save in IE11
• Use try/catch to grab the referrer value instead
• It looks new URL(document.referrer).pathname has been in the code for 5 years https://github.com/alphagov/frontend/blame/1ee93d82ba4f33e95fd5e4136b656d32963d2489/app/assets/javascripts/modules/cookie-settings.js#L98

Why

• /help/cookies is crashing when saving the form in production in IE11

Visual Changes

None