Authentication rewrite

This release introduces breaking changes, be careful when updating.

Users now have usernames, adding admins does not depend on positions anymore.
This is way easier to use and more convenient. Requires client & server configuration update.

Configuration has been updated:

• USER_TOKENS has been removed in favor of USERS (new format)
• MAX_ADMIN_ID has been removed in favor of ADMINS (new format)
• DELETION_SALT has been renamed to SALT (same format)

Some parameters have been changed:

• auth_token has been renamed to token
• deletion_hash has been renamed to key
• user_id has been removed in favor of username and now only exists when a user is authenticated

Please follow https://github.com/Xenthys/ShareXen/blob/master/UPDATING.md for more information.