Add ability to disable use of authentication tokens on ticket links

Add Agents and Users setting pages
Move global settings to System settings

Starting with osTicket 1.8.1, users must receive an email and follow a link
in the email to get access to the ticket. With this new option, the email
verification step can be avoided in osTicket 1.9, because access is now only
granted to exactly one ticket.

- Add tips for fields that were previously overlooked.
- Fix some typos.
- Update tips to adopt the formatting conventions of tips.

Add client registration methods and login required checkbox