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/virtual-scroll/test/basic/main.html

@@ -2,7 +2,7 @@
   <ion-navbar>
     <ion-title>Virtual Scroll{{webview}}</ion-title>
     <ion-buttons end>
-      <button (click)="reload()">
+      <button ion-button (click)="reload()">
         Reload
       </button>
     </ion-buttons>

src/components/virtual-scroll/test/image-gallery/main.html

@@ -2,7 +2,7 @@
   <ion-navbar>
     <ion-title>Virtual Scroll: Image Gallery</ion-title>
     <ion-buttons end>
-      <button (click)="reload()">
+      <button ion-button (click)="reload()">
         Reload
       </button>
     </ion-buttons>

src/components/virtual-scroll/test/sliding-item/main.html

@@ -2,7 +2,7 @@
   <ion-navbar>
     <ion-title>Virtual Scroll{{webview}}</ion-title>
     <ion-buttons end>
-      <button (click)="reload()">
+      <button ion-button (click)="reload()">
         Reload
       </button>
     </ion-buttons>
@@ -22,10 +22,10 @@
     <ion-item-sliding *virtualItem="let item">
       <ion-item>Item: {{item.value}} {{item.someMethod()}}</ion-item>
       <ion-item-options>
-        <button>Archive</button>
+        <button ion-button>Archive</button>
       </ion-item-options>
       <ion-item-options side="left">
-        <button danger>Remove</button>
+        <button ion-button danger>Remove</button>
       </ion-item-options>
     </ion-item-sliding>