Show user initials as a placeholder for the user profile picture

This was introduced in 4.0 and it should be now supported on the app.

See MDL-72305

This might be a bit tricky to implement and maybe we could limit only to the own user profile picture (because for participants for example we don't receive the firstname and lastname separately)

This is the PHP code for the web version:

$output = html_writer::tag('span', mb_substr($user->firstname, 0, 1) . mb_substr($user->lastname, 0, 1),
['class' => 'userinitials size-' . $size]);