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
