`norm` vs `opnorm` transition should be completed

[ericphanson]

Right now, if A is a matrix, then norm(A,p) emits a deprecation warning and calls opnorm(A,p), in line with Julia 0.7. In the next breaking release, we should make norm(A,p) call norm(vec(A), p) as the deprecation warning states, and remove the warning.

[ericphanson]

Step 1: remove norm(A, p) for matrices at all, so it produces an error. This was done in v0.14.0 via #412.
Step 2: re-introduce norm(A, p) as norm(vec(A), p). This will be done in v0.15.0.

This way, hopefully users will not get incorrect answers if they haven't noticed the deprecation warnings (which are off by default on Julia 1.5+).

[odow]

@ericphanson what is the status of this?

[ericphanson]

I will put up a PR to finish it