Add link type for linking to issue tracker issues

[preaction]

Once we get links (#33), we can also start linking to bugtracker issues. Then, later, the website can determine if the issue is closed and show the status of the issue on the report page.

We should add a new link type for tickets. This link type should have the following fields:

• type - This is the type of link. It should be "ticket".
• url - The URL to the ticket. We will use the URL to determine the status of the ticket
• comment - A markdown field for generic comments

These links will likely not be filled in by reporters, but will be filled-in later by users on the website.