ProseMirror adding schema/display for remaining inlines: External links

[Will Sullivan]

ProseMirror adding schema/display for remaining inlines: External links

[Tadhg O'Higgins]

We currently (as soon as #1045 is merged into master) have:

• Schema support for links in ProseMirror.
• Display of links in ProseMirror.
• Rudimentary addition/removal of link markup in ProseMirror.
• Basic tests.

We still need:

• A better UI than prompt for editing the links.
• The ability to add/edit link titles.
• Tests that cover the above interactivity.
• Link validity testing (either just making sure it's a valid URL or actually checking the URL).
• Design & implementation for handling (and displaying) bad links.
• Test that cover the above two points.