[ticket/12099] Fix several issues in path_helper test

PHPBB3-12099
[ticket/12099] Clean some paths before using them

PHPBB3-12099
[ticket/12099] Correctly fix go back to root before prepending the root path

This allows moving app.php outside of phpBB and still work as expected

PHPBB3-12099
[ticket/12099] Clean paths in tests

PHPBB3-12099
[ticket/12099] Break clean_path tests with a simple test

PHPBB3-12099
[ticket/12099] Fix clean_path() ".." stripping when previous directory was "."

PHPBB3-12099
[ticket/12099] Deduplicate path generation

PHPBB3-12099
[ticket/12099] Prepend ./ to path to fix assets

PHPBB3-12099
[ticket/12680] Adds alternative text to contact icon in viewtopic.php

PHPBB3-12680
[ticket/12680] Adds second alternativ text and font-size: 0 to remove text

PHPBB3-12680
[ticket/12680] Adds alternativ text with new variable to contact:

PHPBB3-12680
[ticket/12680] Removes unnessesary $user->lang from CONTACT_USER

PHPBB3-12680
[ticket/12099] Fix correction in path_helper test

PHPBB3-12099
[ticket/12680] Little changes, but some issues are still there

PHPBB3-12680
[ticket/12680] Still some problems with displaying the nickname in ucp

PHPBB3-12680
[ticket/12680] Solves last bugs. viewprofile: contact --> contact nickname

PHPBB3-12680
[ticket/12680] Brings back get_username_string()

PHPBB3-12680
[ticket/12680] Adds variable for guest user

PHPBB3-12680
[prep-release-3.1.0-RC1] Update version to 3.1.0-RC1
[prep-release-3.1.0-RC1] Add migration for 3.1.0-RC1