#64 Make key changes be recorded in issue history

[Chris Puttick]

When changes are made to a submitted issue, these
changes do not show. In the most extreme scenario, a
support agent can change the problem/detail
classification, the summary and the detail, and this
would currently not show in the issue history - this
could be extremely confusing to the person who
reported/submitted the issue.

Ideally, changes to the category/detail and main text
fields should be recorded in the history as
priority/level changes already are.

[Chris Puttick]

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Provisionally marked for 1.0 - if that is too ambitious i.e.
if it's a fair bit of coding, I'll put it back to 1.1.