Incidents: implement `make_username` helper

The justification is to incorporate the `actioned_by` name into the
username in some way, and so the logical thing to do is to abstract
this process into a helper so that it can easily be adjusted in the
future. For now, I've chosen to separate the names by a pipe.

Discord webhook username cannot exceed 80 characters in length, and
so we cap it at this length by default. This is seen as more of an
edge-case, but it should be accounted for, as we're not joining two
names. The `max_length` param is configurable primarily for testing
purposes, it probably should never be passed explicitly.

This commit also provides two tests for the function.

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