* Don't close eventChannel when panicking
The state of the system is completely unknown when panicking and in
general it's not safe to infer whether or not it is safe to close a
channel when in this tate.
* CR feedback
* Spelling
This change implements the same preview behavior we have for
cloud stacks, in pkg/backend/httpbe, for local stacks, in
pkg/backend/filebe. This mostly required just refactoring bits
and pieces so that we can share more of the code, although it
does still entail quite a bit of redundancy. In particular, the
apply functions for both backends are now so close to being
unified, but still require enough custom logic that it warrants
keeping them separate (for now...)
