Alignment issue with OakButton as anchor element

[remy]

Demonstrating through screenshot.

Adding the following CSS to the element (both when an a and button) looks to work, but I've not checked heavily (thus no PR as yet):

display: inline-flex;
align-items: center;

I applied this (in the devtools console) to the .internal-button class to preview if it was fixing the issue (without causing other breakages) and it seemed to (in particular paying attention to vertical alignment when an icon is included).