Update PHPDoc comment in load.php

load.php still has info about includes/checkserver.php, which is now loaded by load-includes.php
Also adjust the See Also section take them out of being inline and add them to the main DocBlock.
Create a LOADED constant, so that a script knows if it was loaded via load.php or not.
Adjust comments in constants.php
Add the PATH constant to constants.php
Update translation strings, including a fix to allow CrowdIn to read the file.
Update the index.php header comment.
Improve PAGE definition by setting it to "index" if the URL parameter doesn't exist.
Remove unneeded whitespace
Add login and registration forms.
Add a first revision of scripts to process logins and registrations, using cookies, $_SESSION, and the PHP password API.
Add a basic upload script, which shows a bare-bones upload form if no file is POSTed.
Add functions to access the database. Support MySQL only at first.