banners: Add new redesigned banner component.

This commit adds the redesigned banner component to the codebase along
with a storybook-style page in /devtools/banners to view and test the
redesigned banner component.

Any banner using the new redesigned styles, requires two classes,
- First, the base `banner` class which defines the structure and
    behavior of the banner.
- Second, a modifier class like `banner-info` which defines the styles
    for the particular banner type.

The navbar alert banners also have a custom class `navbar-alert-banner`
which is used to define the specific style and structure for these
banner types.

This commit also makes the `banner`, `action-button` and `icon-button`
components into handlebar templates to maintain consistency in their
usage in the codebase.

web/templates/components/action-button.hbs

@@ -0,0 +1,6 @@
+<button class="{{#if custom_classes}}{{custom_classes}} {{/if}}action-button action-button-{{type}}-{{intent}}" tabindex="0">
+    {{#if icon}}
+    <i class="zulip-icon zulip-icon-{{icon}}"></i>
+    {{/if}}
+    <span class="action-button-label">{{label}}</span>
+</button>

web/templates/components/banner.hbs

@@ -0,0 +1,13 @@
+<div {{#if process}}data-process="{{process}}"{{/if}} class="{{#if custom_classes}}{{custom_classes}} {{/if}}banner banner-{{intent}}">
+    <span class="banner-label">
+        {{label}}
+    </span>
+    <span class="banner-action-buttons">
+        {{#each buttons}}
+            {{> action-button .}}
+        {{/each}}
+    </span>
+    {{#if close_button}}
+        {{> icon-button custom_classes="banner-close-button" icon="close" intent=intent}}
+    {{/if}}
+</div>

web/templates/components/icon-button.hbs

@@ -0,0 +1,3 @@
+<button class="{{#if custom_classes}}{{custom_classes}} {{/if}}icon-button icon-button-{{intent}}" tabindex="0">
+    <i class="zulip-icon zulip-icon-{{icon}}"></i>
+</button>