Rethink Gutenberg implementation

[seankwilliams]

The current Gutenberg implementation was done a few months after the initial release of WordPress 5.x and there wasn't great developer documentation around Gutenberg. Eventually, that should be revisited and improved so that Modern Footnotes works similarly to other plugins that allow inline editing within blocks. (Or, is there a use case where it makes sense for a footnote to be its own block?)

Related topic: https://wordpress.org/support/topic/add-support-for-multi-line-footnotes-and-image-footnotes-in-gutenberg/
Realted issue: #39

[Stimson]

Agree it would be nice to have a cleaner Guttenberg implementation. Right now my team takes a word doc, runs macros to insert footnote contents inline, and then pastes that into blocks -- we do that largely because it takes a lot of tedious clicking to do the FN insertions manually. But something that a paste from Word and made the FNs in a block would be excellent.