Switch to `b` and `i` elements for bold and italics and fix annotation imports

[camdendotlol]

Summary

• updates the Leaf element to render bold with b tags and italics with i tags
• updates the deserialize script to support italics with both em and i tags, and add support for b tags for bold (along with the already-existing strong tags)
• fixes an issue where a plain text annotation (i.e. without any HTML tags) would produce a bad Slate node that would crash Slate
• fixes an issue where an HTML annotation would be wrapped in two layers of brackets, breaking both the Slate editor and the display logic

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