buttons: Add redesigned button styles.

This commit adds the redesigned button styles to the codebase along with
with a storybook-style page in `/devtools/buttons` to view and test the
redesigned button component.

The redesigned button component, uses the `action-button` class to
follow Zulip's no-abbreviation policy, and to avoid conflicts with the
pre-existing `button` and bootstrap `btn` classes.

A button using the new redesigned styles, required two classes,
  - First, the base `action-button` class which defines the structure
  and behavior of the button.
  - Second, a modifier class like `action-button-primary-neutral` which
  defines the styles for the particular action button type.

templates/zerver/development/dev_tools.html

@@ -79,6 +79,11 @@
                     <td>None needed</td>
                     <td>Test incoming webhook integrations</td>
                 </tr>
+                <tr>
+                    <td><a href="/devtools/buttons">/devtools/buttons</a></td>
+                    <td>None needed</td>
+                    <td>Test button styles</td>
+                </tr>
             </tbody>
         </table>
         <h2>Useful management commands</h2>