Switch "Edit in Plunker" to use POSTs

[chirayuk]

"Edit in Plunker" used to use POSTs to create pre-seeded examples.  At some point, this was switched over to a scheme that relied on hardcoded regex matching and URLs that are stored on plunker's servers.  This leads to breakages such as issue #7166.  It also makes it really hard for someone who wants to maintain to code to figure out how the whole thing fits together.  We should go back to using POSTs to create plunkers and remove this tight coupling / brittleness.

[chirayuk]

See also #7166 (comment)

[chirayuk]

If nobody ends up picking this, I'm happy to do it.  I may just not get to it anytime soon so just ping me if/when this becomes urgent.

[petebacondarwin]

I will sort this out this week.