Create Pledge: Make form functional (#31)

This broadly enables & styles the pledge forms across the frontend and wp-admin.

- Switches field `readonly` control to a variable called `$readonly`
- Create `PledgeMeta\get_pledge_meta()` which will fetch data from `$_POST`, a pledge post, or defaults.
- Add Number of Employees field
- Update form content to match mockup:
  - fields now have help text
  - success message uses content from mockup
  - logo field is moved to the "org info" section
- Style form in theme, add some similar styles to the admin

Fixes #7, fixes #28

plugins/wporg-5ftf/views/form-pledge-manage.php

@@ -7,15 +7,19 @@ use function WordPressDotOrg\FiveForTheFuture\get_views_path;
 /** @var bool  $updated */
 ?>
 
-<form id="5ftf-form-pledge-manage" action="" method="post">
+<form class="pledge-form" id="5ftf-form-pledge-manage" action="" method="post">
 	<?php
 	require get_views_path() . 'inputs-pledge-org-info.php';
-	require get_views_path() . 'inputs-pledge-org-logo.php';
-	require get_views_path() . 'inputs-pledge-org-email.php';
 	require get_views_path() . 'inputs-pledge-contributors.php';
+	require get_views_path() . 'inputs-pledge-org-email.php';
 	?>
 
 	<div>
-		<input type="submit" id="5ftf-pledge-submit" name="action" class="button button-primary" value="<?php esc_attr_e( 'Update Pledge', 'wporg' ); ?>" />
+		<input
+			type="submit"
+			id="5ftf-pledge-submit"
+			name="action"
+			value="<?php esc_attr_e( 'Update Pledge', 'wporg' ); ?>"
+		/>
 	</div>
 </form>