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/inputs-pledge-contributors.php

@@ -1,9 +1,9 @@
 <?php
 namespace WordPressDotOrg\FiveForTheFuture\View;
 
-/** @var bool $editable */
-/** @var array $data */
 /** @var array $contributors */
+/** @var array $data */
+/** @var bool  $readonly */
 ?>
 
 <?php if ( empty( $contributors ) ) : ?>
@@ -14,14 +14,14 @@ namespace WordPressDotOrg\FiveForTheFuture\View;
 	</label>
 	<input
 		type="text"
-		class="large-text"
 		id="5ftf-pledge-contributors"
 		name="pledge-contributors"
 		value=""
 		required
+		aria-describedby="5ftf-pledge-contributors-help"
 	/>
-	<p>
-		<!-- Instructions for inputting wporg usernames -->
+	<p id="5ftf-pledge-contributors-help">
+		<?php esc_html_e( 'Separate each username with a comma.', 'wordpressorg' ); ?>
 	</p>
 </div>