[WiP] [FIX JENKINS-41966] load and save

[kzantow]

This depends on corresponding changes in jenkinsci/blueocean-plugin#886

Here's a quick preview: https://www.youtube.com/watch?v=clnUzUvq--0

TODO:

• dialog aspect ratio
• title not present by default bug
• default text to 'Updating Jenkinsfile' if nothing provided
• provide a popup that they'll lose their work when leaving the page
• italicize branch name on screen
• error handling -
• make sure the edit buttons only shows up for github
• swap buttons with load/export if go directly to editor
• sort extension points somehow - alphabetically? (do this last; already solved with new extension mechanism)

[michaelneale]

nice!

[michaelneale]

Amazing @kzantow !
I think this could go in. There are things (in tickets) found, but all minor except for:

https://issues.jenkins-ci.org/browse/JENKINS-42646
for empty repo.

Otherwise:

If this goes in, we can cut a blueocean-plugin so this could be published ready for RC-ish.