Include sharable link in "favorite sessions" email

[timiwahalahti]

The "favorite sessions" email includes the sessions the user chose, it'd be helpful to include the link, so that they can easily apply those sessions to their phone or other device, and so that they can easily get a much better view of their personalized schedule on the website.

This PR adds that link to that email.

For ease, let's get the schedule page by slug. This way we do not need to pass the page ID just for this, but we neither blindly trust the input sent to REST API as a URL that belongs to the WordCamp site.

Fixes #186

Props @iandunn

Screenshots

How to test the changes in this Pull Request:

1. Go to WordCamp website schedule page
2. Favorite some sessions
3. Send the email
4. Open the link in email, favorite sessions should be indicated

[timiwahalahti]

Lint errors (phpcs) are coming from rule that I don't get.

'validate_callback' => function( $value, $request, $param ) { is way better than
'validate_callback' => function ( $value, $request, $param ) { 🤔

[pkevan]

Lint errors (phpcs) are coming from rule that I don't get.

'validate_callback' => function( $value, $request, $param ) { is way better than 'validate_callback' => function ( $value, $request, $param ) { 🤔

For some reason PHPCS is treating the function keyword as a function declaration instead of being an anonymous function as it is in this case.

I can see two ways of this being used in the wc-post-types/inc/rest-api.php file, including one with a space, perhaps to get over this error.