Create a Pledge: Upload a logo (#45)

* Allow image uploads

* Add the logo as "generated" meta

This will be attached after we've processed the `$_FILES` object into an upload.

* Load image into media library, attach to the submitted pledge as a featured image.

* Move where image handling happens so we can pass back errors

* Pass back error if upload fails

* Create a validation function to do basic checking of form data

* Move logo uploading to a separate function, and rely on core checks for mime & filesize checking

* Tweak function name to be more clear

* Remove bmp, tiff, and ico file types

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

@@ -7,7 +7,7 @@ use function WordPressDotOrg\FiveForTheFuture\get_views_path;
 /** @var bool  $updated */
 ?>
 
-<form class="pledge-form" id="5ftf-form-pledge-manage" action="" method="post">
+<form class="pledge-form" id="5ftf-form-pledge-manage" action="" method="post" enctype="multipart/form-data">
 	<?php
 	require get_views_path() . 'inputs-pledge-org-info.php';
 	require get_views_path() . 'manage-contributors.php';