Make the markup-view connect to multiple remote fission iframes for Fission.

Since bug 1560200, the markup-view can now connect to remote <browser> elements in the browser toolbox.
This bug is about doing the same but for fission remoted <iframe>s in the normal toolbox.

Tentatively moving all bugs whose summaries mention "Fission" (or other Fission-related keywords) but are not assigned to a Fission Milestone to the "?" triage milestone.

This will generate a lot of bugmail, so you can filter your bugmail for the following UUID and delete them en masse:

0ee3c76a-bc79-4eb2-8d12-05dc0b68e732

Blocks enabling Fission in Nightly (M6).

I'm not sure there is anything specific to be done in the markup view, but we want to make it work in M2.
It most likely depends on bug 1598022 and its followups.

I tested the markup-view again on nightly 75, with fission ON, and it really seems to work fine. So, yes, there are probably a lot of follow-ups, and some things like the node picker and highlighters don't work well, but those are all being tracked in other bugs. So I think it's probably ok to close this one. The main work has already been done, and the rest will be done elsewhere anyway.