Implemented registration submission

Fixed missing config.ini property causing issues when loading config.
Define allowEntry earlier in pages and start session before
src/includes/pageTemplate.php is included. $CFG_FILE path in
src/includes/config.php will point to the correct file when executing a
script outside the src/includes folder. Implemented server-side
functions in PHP that parallel the JavaScript equivalents in
src/register.js. Implemented src/register.php#commit so that fields are
validated on the server side, new users are committed to the database,
new users are associated with the session, and page redirects work.
Unique nicknames

Copied and pasted some duplicate email checking code in src/register.php and src/register.js to also support duplicate nickname checking. Added index for `users`.`displayname` column for faster nickname check querying. Restrict `users`.`displayname` to a maximum of 20 characters to match registration form condition. Updated database.sql so that existing tables are dropped before new ones are created. Use mysqli_stmt::bind_result and mysqli_stmt::fetch instead of mysqli_stmt::get_result and mysqli_result::fetch_array to support hosts without mysqlnd.
Preparation for full user login support

Store nickname of logged in user in session. Define allowEntry earlier so that including includes/loginFunctions.php will not cause a 403. Fixed includes/loginFunctions.php#loadCookie's broken query and method call to a null object. Use isset instead of array_key_exists. Give HTTP 403 with improper GET/POST keys in src/register.php#commit. Give HTTP 403 when opening src/register.php while already logged in. Changed id of Register button in src/register.php and renamed src/register.css to src/formpage.css so that its rules will be able to be used on the login page too. Added a checkbox for persistent login and a link for password reset to login form on header. Added maxlengths to fields on login form in header. Changed the form id of the header's login form so that its CSS rules could be used for the user profile box in the header when a user is logged in.
Implemented login submission and login form

Pressing the Login button on the header or on src/login.php will now do
the backend stuff for cookies and sessions. src/login.php without any
POST variables will display a full page login form for when the user
mistypes their email address or password. Header's input#persistent id
changed to input#remember to prevent conflict with input#persistent in
src/login.php. Swap out the login box for a user profile box in the
header when a user is associated with the session.