Move 'Skip to Main' link to header template

[cjstevens78]

During a recent accessibility review on Croydon via the AXE auditor - it was flagged that the 'Skip to Main' link should be contained within a landmark. As this change would benefit the localgov theme im adding the change here instead of within the Croydon theme.

[cjstevens78]

PR: #147

[paulpopus]

I'm updating this issue to match the discussion from the PR above.

Old title: Contain 'Skip to Main' link within a Landmark

To do: Move skip to main link from nav to header and remove the nav wrapper.