UL&S: Add visual blur to Prologue button view

[mindgraffiti]

Use a UIVisualEffectView with .systemChrome style for iOS 13+ and regular for iOS 12. This keeps our button view designs in line with other areas. For example, it's used in the "What's New" feature:

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[ScoutHarris]

@mattmiklic @mindgraffiti I'm a bit confused as to what this will accomplish. On the Prologue, the carousel pages sit on top of the button view, never going under. The new designs appear to be the same way. The blur made sense on the Login Epilogue since My Sites can scroll, thus achieving a blur. But here, there won't be anything to blur.

[mindgraffiti]

@ScoutHarris oh that's true. Even the i5 prologue designs don't include a visual blur. @mattmiklic I propose closing this issue, unless there is an i6 for prologue designs expected that would incorporate a visual blur.

[mattmiklic]

My expectation for the prologue was that the background color would continue behind the button view, and the button view's system material would allow a bit of the color to show through. In @osullivanchris's latest prologue work the circular background pattern could continue underneath the system material too. It would be a quite subtle effect but I think the value in it would be matching the general background color and border style of the Epilogue and What's New button views; this would be the only screen that didn't use it if it weren't updated, I believe.

[osullivanchris]

Yeah that's what I was thinking too, that the background and pattern would continue to the bottom of the screen, but behind the buttons & blurry background

[ScoutHarris]

Ah, I see. Thanks @mattmiklic & @osullivanchris . Blur shall be added!