try to optimize edit theme form (bug 855619)

Changes:

- Reduces one call to ```get_tags``` by storing it in the form init and updating it in the form save.
- Checks whether add-on properties (name, desc, slug) have been modified before saving the add-on.
- When checking whether ```TransField```s have been modified, use utility function that compares TransField in initial data against cleaned data of the form.