Index accessiblity updates

[CWDamm-Kint]

This PR makes a number of accessibility related changes.

• adds an accessibility statement provided by Jersey as a plugin. The plugin is created and the permissions are assigned as part of two project migrations.
• adds a missing aria label
• provides a background to the captions on the index page
• changes the social media link images so that they are no longer background images, but regular images with alt text.
• makes further css changes to increase the contrast on various elements
• adds alt text to the jersey heritage logo in the header

Closes: #36

[CWDamm-Kint]

@samuelscandrett - I've realised there's still WCAG Contrast checker issues to address. Hold off on the review for now and I'll give you a nudge when they're done.

[samuelscandrett]

lgtm!