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/content/content.ts

@@ -61,7 +61,7 @@ import { isTrueProperty } from '../../util/util';
  *       </ion-toolbar>
  *     </ion-header>
  *     <ion-content>
- *       <button (click)="toggleToolbar()">Toggle Toolbar</button>
+ *       <button ion-button (click)="toggleToolbar()">Toggle Toolbar</button>
  *     </ion-content>
  * `})
  *
@@ -85,7 +85,7 @@ import { isTrueProperty } from '../../util/util';
  *
  * @Component({
  *   template: `<ion-content>
- *                <button (click)="scrollTo()">Down 500px</button>
+ *                <button ion-button (click)="scrollTo()">Down 500px</button>
  *              </ion-content>`
  * )}
  * export class MyPage{