Implement inter-team comment soft assign

[zanerock]

'Soft assignment' is always available to both team and non-team members. It's the only way for non-team members to put any kind of claim on an issue. Rather than assign the issue, it adds a comment 'XXX is working on this.' to the issue.

Use the github 'check assignee' API method to see if the current user can be assigned to the issue. If so, we use hard assignment. If not, then we fall back to soft assign. If '--noassign' is passed in, then we always use soft assign.

2 tests:

• non-assignable user triggers soft assign; comment appears in issue
• assignable user issues '--noassign'; assignment is not made and comment appears in issue