Convert login URL to constant

Rationale: publicly-accessible web systems are often subject to brute force attacks (Wordpress, for example, suffers terribly due to its ubiquity). Customisable login URLs are therefore a logical step towards reducing the volume of brute force attacks for any web system.

This pull req defines a LOGIN_URL constant in constants.php, and replaces any hard-coded references to the 'login' url with the constant.

Out of the scope of this pull req, but things to consider for Bonfire devs wishing to further secure their site:
1) Disabling the 'controller' routes for login/register in routes.php:
```
$route['users/login']					= ''; // Disable standard login URL
$route['users/register']				= ''; // Disable standard register URL
```

2) Changing their SITE_AREA constant from 'admin' to something more unique. Little point in changing your login URL if 'admin' will take you there too ;) In constants.php:
```define(SITE_AREA, 'super_secret_admin_url')```

3) Disabling controller routes for admin (only if you've changed SITE_AREA!) in routes.php
```$route['admin/(:any)']					= ''; // Disable standard admin controller URLs```