Hyperlink Support in Slate 0.57

[jolanglinais]

We need support back for hyperlink editing.

Needs to be done in conjunction with:

• Improved Hyperlink UI #293

Can be done after:

• Migrate to Slate 0.50.x #297

Related Issues in markdown-editor:

• Keyboard Shortcut for Links #249
• Removing link followed by Undo shortcut crashes demo #272
• Selecting list/quote option while link popup is open crashes demo #266
• Make "Insert a link" shortcut ⌘+K on mac and CTRL+K on Windows #296

Related Info:

• Issue in Slate.
• Related issues discussed in Slate's Slack (these may no longer exist): one, two, three.
• Hyperlink example in Slate docs
• Useful snippets

Main files involved:

src/RichTextEditor/components/withLinks.js
src/RichTextEditor/FormattingToolbar/HyperlinkButton.js
src/RichTextEditor/FormattingToolbar/HyperlinkModal.js