Refactor bug subscription change to use the IBugChange API

All bug subscriptions and unsubscriptions should be recorded using the
BugChange API.

The fix for bug #155290 added unsubscribe handling using the BugChange
API, but subscribe handling has not been changed. Additionally, what
is recorded is a bit of a mess right now:

  On subscribe:

    * What changed: bug
    * Old value: <empty>
    * New value: <empty>
    * Message: added subscriber Fred Bloggs

  On unsubscribe:

    * What changed: removed subscriber Fred Bloggs
    * Old value: <empty>
    * New value: <empty>
    * Message: <empty>

I think this should be changed to:

  On subscribe:

    * What changed: subscriber added
    * Old value: <empty>
    * New value: fredbloggs
    * Message: <empty>

  On unsubscribe:

    * What changed: subscriber removed
    * Old value: fredbloggs
    * New value: <empty>
    * Message: <empty>

Some justification. Custom formatting will be added to the activity
log, and is already being added to log interleaving, so it makes sense
for "what changed" to be a simple static value. And before the
formatting is put in place, using "old value" and "new value" as above
is close to how other changes are represented in the activity log.