refactor(button): add ion-button attribute and icon attributes to style buttons

BREAKING CHANGES:

- `<button>` becomes `<button ion-button>`
- `<a button>` becomes `<a ion-button>`
- `<button ion-item>` does not get the `ion-button` attribute
- Buttons inside of `<ion-item-options>` do get the `ion-button`
attribute
- Removed the `category` attribute, this should be passed in
`ion-button` instead.
- Button attributes added for icons in buttons: `icon-only`,
`icon-left`, and `icon-right`

closes #7466

src/components/infinite-scroll/test/basic/index.ts

@@ -46,7 +46,7 @@ class E2EPage1 {
 
 
 @Component({
-  template: '<ion-content><button (click)="navCtrl.pop()">Pop</button></ion-content>'
+  template: '<ion-content><button ion-button (click)="navCtrl.pop()">Pop</button></ion-content>'
 })
 class E2EPage2 {
   constructor(public navCtrl: NavController) {}

src/components/infinite-scroll/test/basic/main.html

@@ -13,7 +13,7 @@
     InfiniteScroll is enabled: {{enabled}}
   </p>
 
-  <button (click)="toggleInfiniteScroll()" block>
+  <button ion-button (click)="toggleInfiniteScroll()" block>
     Toggle InfiniteScroll Enabled
   </button>