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

themes/wporg-5ftf/css/objects/_objects.scss

@@ -8,5 +8,6 @@
 @import "../../../pub/wporg/css/objects/notices";
 @import "buttons";
 @import "image";
+@import "pledge-form";
 @import "pullquote";
 @import "site-header";