Enh 0002811: Anonymous mode issues (auto login for anonymous users + other issues).

This enhancement is based on patch provided by nepto (thanks Nepto!).

M signup_page.php

• Removed the [ Go Back ] link.

M print_bug_page.php

• Remove the [ Go Back ] link (not really related, but I did it to be consistent with singup page)

M core/html_api.php

• Updated the menu to remove logout when user is anonymous.
• Added "Anonymous | Login | Signup" where user can login or signup.

M core/authentication_api.php

• Allow auto-login if anonymous login is enabled.

M lang/strings_english.txt

• Added 'Anonymous' string.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2299 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9