Lock incident issues by default

Having customers comment on incident issues makes it harder for the Engineer OnCall, Incident Manager and other stakeholders to see the relevant information necessary. In the interest of fastest Time to Resolution, this MR proposes locking incident issues by default. This still keeps them visible to customers, preserving the value of Transparency.

Question for feedback: Do we want to acknowledge this change in the issue template or otherwise communicate this out? I thought about adding a line to external customers affected by a particular incident, but was wary of flooding support with tickets.