Print email link as button with envelope icon

In the View Issue Details section of view.php page, user names were
displayed with a mailto: link. This is not consistent with the way these
links are printed in other locations, where it goes to the User
Information page (view_user_page.php).

With this commit:

• username now links to view_user_page.php
• the mailto: link is available as a button next to the username

Fixes 0025686