Add support for multiple Actions in Mail Notifications

[ahinkle]

This PR adds support for multiple action method calls in mail notifications.

Currently, calling ->action() more than once would only render the last button, without any exception or warning.

For example, given the following mail message:

return (new MailMessage)
    ->line('You have been invited to join a team!')
    ->action('Accept', 'https://example.com/accept')
    ->action('Reject', 'https://example.com/reject');

It currently renders with only the last action on the chain:

With this PR, it allows multiple action methods to render:

Secondly, it enables mixing line and action calls in any order.

return (new MailMessage)
    ->line('Intro line 1')
    ->line('Intro line 2')
    ->action('Accept', 'https://example.com/accept')
    ->line('Outro line 1')
    ->action('Reject', 'https://example.com/reject')
    ->line('Outro line 2');

This supports backwards compatibility with no breaking changes.