The review dialog now displays a delete icon next to all deletable

comments (diff comments and attachment comments, currently). Previously,

the only way to delete a comment was to delete it from the review UI

where it was created. However, with general comments coming soon, there

was no review UI to manage them and delete them. Hence, it makes sense

for all deletable comments to be deletable from within the review

dialog.
JS Unit tests have been added for deletion of diff comments, attachment

comments, and screenshot comments.
This patch also contains some minor JS cleanups:

- RB.BaseResourceModel.ready now ensures its success and error

  options are functions before bdinging them.

- Replaced a binding of self = this and calling a function by binding

  said function to the correct context.

- Cleaned up some assignments in RB.BaseResourceModel._finishDestroy

  so that it only does one set() call instead of several.

Ran JS tests.
Deleted attachment comments and diff comments from the review dialog.

  It worked as expected.
Manually verified that the deletion confirmation works in both cases.