Problem based learning : replace custom button classes

Problem

The current buttons on the homepage and investments are being styled using custom classes:

<a href="https://wellcome.ac.uk/funding/guidance" class="button button--full button--island">I'd like to manage my grant</a>

class="button button--full button--island"

Delete these classes and then add existing classes, use existing classes to style the links and then delete the classes within the code base