LEDE-2616: Header and Footer Enhancements

[efuller]

Summary

• Adds in a filter to enable/disable ads support
• Fixes a bug that was causing header images to not display
• Adds in a custom 'Manage Subscription Preferences' link -- in the footer block

Notes for reviewers

• Issue Make the Footer Block Customizable #135 has been created to allow users to be able to customize the output of the footer block.
• Issue SendGrid Suppression Group ID or Custom Unsubscribe URL #136 has been created to allow users to be able to configure a suppression group id or a custom unsubscribe url.
• Will bump version after PR has been approved.

Changelog entries

Added

• a filter to enable/disable ads

Changed

• footer markup to allow for a custom subscription management link

Removed

Fixed

• header images not displaying
• footer settings not displaying

Ticket(s)

https://alleyinteractive.atlassian.net/browse/LEDE-2616

[kevinfodness]

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