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-org-email.php

@@ -1,23 +1,26 @@
 <?php
 namespace WordPressDotOrg\FiveForTheFuture\View;
 
-/** @var bool $editable */
 /** @var array $data */
+/** @var bool  $readonly */
 ?>
 
 <div class="form-field">
-	<label for="5ftf-pledge-email" class="screen-reader-text">
-		<?php esc_html_e( 'Email', 'wordpressorg' ); ?>
+	<label for="5ftf-pledge-email">
+		<?php esc_html_e( 'Administrator Email', 'wordpressorg' ); ?>
 	</label>
 	<input
 		type="email"
-		class="large-text"
 		id="5ftf-pledge-email"
 		name="org-pledge-email"
-		value="<?php echo esc_attr( $data['pledge-email'] ); ?>"
+		value="<?php echo esc_attr( $data['org-pledge-email'] ); ?>"
 		required
-		<?php echo ( $editable ) ? '' : 'readonly'; ?>
+		aria-describedby="5ftf-pledge-email-help"
+		<?php echo $readonly ? 'readonly' : ''; ?>
 	/>
+	<p id="5ftf-pledge-email-help">
+		<?php esc_html_e( 'This email will be used to verify your organization’s contribution profile, and later manage any changes.', 'wordpressorg' ); ?>
+	</p>
 
 	<?php if ( is_admin() ) : ?>
 		<?php if ( $data['pledge-email-confirmed'] ) : ?>