TWIW newsletter links should use enough contrast

[thibaudcolas]

At the moment, the links in the This week in Wagtail newsletter don't provide enough contrast to pass accessibility checks.

Screenshot:

The font size should be increased, or the colors should be changed to ones that are higher-contrast over white.

Relevant template (thanks @kaedroho for the find!): https://github.com/wagtail/wagtail.io/blob/master/wagtailio/newsletter/templates/newsletter/newsletter_page.html

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Initially raised on Slack.