Some cleanup & added license reference

Removed unused constant, added reference to license at head of every
file, simplified generation of link to non-ASB status list settings.
2.0 version number, display order incrememented

Set version number to 2.0, and set the display order of settings to be
automatically generated using an incremented number when the plugin is
installed.
Added placeholder settings

Added tentative Admin CP settings for new Steam login functionality
(though they currently don't do anything). Did not add them to the
upgrade script yet.
Added template and edits for Steam login button

1. Added Sign in through Steam button images.
2. Created template for Steam login button.
3. Added template edit to eventually allow adding Steam login button to
the welcome block.
4. Added some extra descriptors to make the code easier to read.
Steam login & initial work on register

1. Added LightOpenID library.
2.  Added function for redirect to Steam Community to authenticate when
signing in through Steam.
3. Initial work on function for validating Steam sign in and registering
user if first time signing in through Steam.
4. Added function for checking user's current forum name & avatar
against Steam's and syncing them if they differ.
5. Added database fields for User CP settings that will control syncing.
6. Special characters are no longer removed from Steam names and game
names (this needs to be tested).