Create a new ButtonStyle for the 300x40 blue buttons

We use these buttons everywhere in the login and onboarding flows, and the styling is all a mess of copy-paste where I implemented it once and then just pasted the same set of view modifiers onto every instance that I wanted to look like this. Our philosophy is very much "YAGNI over DRY" but this level of duplication is taking it way too far.

Much better to create a proper ButtonStyle to encapsulate this style, and then apply it to all the places where we use it.