Allow the fine-grained configuration of notification settings for

each section and reword the help texts quite a bit. This will
later be used to collect the correct set of email addresses to
notify a particular audience about changes in a particular section.

Notice that a project admin will have to explicitely opt-in for
"Others" notifications, i.e. unless the checkbox is checked, existing
email addresses won't be notified anymore. This is surely debatable
for existing setups, but makes much more sense for new setups.
Eventually we'll write a small migration script to add the specific
enabled setting for those (existing) projects that have a non-empty
mail list configured.

This commit has been sponsored by SciLab.

NEWS.mdtext

@@ -12,6 +12,8 @@ by the friendly folks from Scilab <http://www.scilab.org/>!
   implementations of this web hook as this setting is likely to be removed
   in future versions of Indefero.
 - Indefero now needs PHP's zip module which is not enabled by default.
+- Existing email notifications now have to be explicitely activated in the
+  project's administrative area.
 
 ## New Features