Enable string lists

[ohmoses]

Closes #389

String fields can now have a flag publish.list, which makes the field display in the edit view as a list of items which can be added, removed, and reordered with drag-and-drop.

Behaves same as Google Keep:

• Enter (Ctrl+Enter if publish.multiline) to move everything after the cursor to a new item below.
• Delete (if cursor is at the end of the item) and backspace (if cursor is at the beginning) to concatenate with next and previous item, respectively.
• Arrow up (if cursor at the beginning of an item) or arrow down (if cursor at the end) to move between items.
• Typing into the last input (the "add item" input) immediately creates the item and adds a new empty "add" input below.

Fixed bugs

• On all plain string fields (not rich editor), the cursor would jump to the end of the input after each keystroke → fixed
• publish.display.link being a string (e.g. https://twitter/{value}) didn't work as intended; the preview link ("Open in new window") would only show if the value itself started with "http://" or "https://" → fixed

As a note, I also enabled using values that don't start with http(s) even without a display.link template; in that case, http:// is added at the front, so that the user can write something like github.com and still get a preview link.

Future to-dos

• Auto-resized multiline textarea (heightType = 'content') with one line of text auto-sizes to 32px height, more than the height of the one line (20px). This is a bit ugly in lists, should be fixed eventually (but doesn't feel high priority).
• I wanted to hide the buttons (link preview, remove) unless the item is focused, dragged, or hovered over; this created a bunch of complications, but I would maybe like to revisit that at some point, after we finish the more urgent things. I left a little bit of the relevant CSS there commented out.
• If the user scrolls the page while dragging the item, it creates a bit of a funny effect, but I don't think it justifies putting the time in to fix it at the moment.
• The list logic now makes up something like half of the code of the component; possibly it could be factored out into a component of its own, just something easy like

  if ((publishBlock && publishBlock.list) || Array.isArray(value)) {
  return <StringArrayEditor {...this.props} />
  }

• Tests!

[ohmoses]

Oh, while I was touching the code, I changed a couple of instances of someFn.call(this, ...) to someFn(...), it evaluates to the same thing when the function is not called as a method. Hope that's okay.

[eduardoboucas]

Added some minor style comments. Haven't tried the functionality yet. I think I'll try to change the CI box to use the repo API first. 🤞

[ohmoses]

@davidmacp Awesome, thanks! :tada: