Added named_scope in Issue and ActsAsWatchable replacing custom finder method declarations in model classes and custom find queries in views.

More specifically this patch introduces the following changes:

• Replaces @watched_by@ class method in ActsAsWatchable with a @named_scope@ of the same name (so no API breaking)
• Adds @Issue::recently_updated@, @Issue::with_limit@ and @Issue::on_active_project@ which are optional but handy. If they seem reduntant they can be removed and update the views accordingly (see next bullet)
• Removes DB logic from partial view @my/blocks/_issueswatched.rhtml@ which falls back to new named scopes. It would be even better if this thing was inside the controller (that is the right place) but because of the modularity of my/page (due to the blocks) I didn't touch it.

plus:

• Tests for introduced changes

and

• Minor documentation for introduced changes

Patch is against trunk r2250.

Note: If you find it useful I could also refactor other parts of redmine, replacing custom finders with named scopes (if possible) and extracting model logic from views. Just let me know, or simply apply this patch :)