Enhancement: subscribe to components in Trac

Enhancement request for Django's Trac: add a way to subscribe to notifications on components. ([forum discussion](https://forum.djangoproject.com/t/subscribe-to-trac-notifications-on-a-component/32529/2))

As a maintainer of a third-party library who uses Django's email APIs, I wish I could automatically be notified about all (future) tickets with "Component: Core (Email)."

I imagine this would involve:

* A new Trac user preference to manage a list of watched components
* Some way to control those notifications

For that second part, I can think of at least two approaches:

1. Auto-add CC on watched components

Whenever a ticket's component is modified, automatically add a CC for each user watching the new component.

Pro: can easily "mute" an individual ticket within a watched component, by just removing yourself from the CC

Con: might generate a lot of "CC modified" notification spam to *other subscribers* every time a ticket's component is changed or a user adds a component to their watch list

OR

2. New subscription rule for "watched component" notifications

In Trac's [notification preferences](https://code.djangoproject.com/prefs/notification), allow something like this mockup (where "watched component" would link to the preference for managing the watch list):

Pros/cons: reverse of the first approach

I don't have a strong preference (and don't know which would be easier).

Incidentally, it might also be helpful to watch keywords as well as components. (But my own current needs would be covered by components.)