Improve text and links about Trac notifications and preferences

[sabernhardt]

Related: #314, #7180

When people comment on Trac tickets, finding how to edit notifications they receive can be difficult.

1. The ​"Trac UI Preferences" link is in the footer. I would not consider notifications as part of the UI, and these preferences are specific to each Trac. I suggest editing the link text to fit each Trac:
   <a class="preferences-link" href="${href.prefs()}">[Meta] Trac Preferences</a>
   
   
2. When you arrive at the General tab, it talks about ​configuring notifications, but that link goes to the page to sign up for emails about tickets by component, milestone, focus, etc. Replacing the sentence and the generic "here" link text could help clarify that link's purpose:
   
   

   To configure notifications about tickets that might interest you, ​choose components to follow.
   

   
   
3. The General tab could include a new sentence about activity notifications:
   To change when you receive updates on your tickets, <a href="/prefs/notification">adjust notifications about ticket activity</a>.