Warn when leaving page with unsaved comment
Fix ARRAY(0x..) keywords when cloning bugs
Put cloned bug title into bug from which it was cloned
Experimental implementation of select fields as hinted editboxes
Begin implementation of "custom" editable object classes.

* Separate 'class' and 'field' entities. Previous Fields were a combination of both a Field and a Class.
* Turn Component, Version, Milestone, Keywords, OS, Platform, Status and all custom select field value classes into generic classes.
  Remove most of their custom DB interaction code.
* Remove separate Milestone and Version CRUD user interfaces, edit them using generic UI.
* Allow to view history of generic objects.

TODO:
* Editor for generic classes and their fields.
* Deal with possible form field name collisions.
* Support search on generic objects.
* Add "display" pages for generic objects and decide how they should
  relate to object edit pages (for example, it's the same page for bugs).
* Add object-level and field-level permission system.
* Support generic "autocompleted editbox" view of select fields,
  without the need to dump all values into client JS.