AccountProxy should not be serialized

Problem/Motivation

Some forms desire to render the name of the currently logged in user (e.g., the comment form). If the form is stored in the cache, then the AccountProxy gets serialized. This currently does not seem to have any bad effects, but it does not make much sense to serialize the proxy either. Affected code typically looks like this:

$form['#theme'] = 'username';
$form['#account'] = $this->currentUser;

Proposed resolution

?

Remaining tasks

User interface changes

API changes