Add ability to select/change Submitted role assignment style

[kierio04]

• Add optional argument in /$start-task called "role_style" accepting values 0 (on submission) or 1 (on deadline) as valid to enter
• If role_style is 0 then the Submitted role is assigned when the user submits their first file
• If role_style is 1 then the Submitted role is assigned at the deadline (for speed tasks, this is the user's custom deadline)
• If this argument is not given, it defaults to 0 when speed_task is 0, and it defaults to 1 when speed_task is 1 (this is how it works currently)
• Add /$set-role-style command with compulsory "role_style" argument accepting values 0 or 1