these have been broken for a while due to the data loss when insert's server failed, and were also deprecated upon the addition of custom server emoji.
they were meant to be temporary anyway, and other clients (including the new web app) don't implement them, so let's remove them to avoid confusion.
* replaces old mentions with avatar and display name
* renders things directly through React
* replaces most of the markdown hacks with custom AST components
* adds a tooltip to codeblock "copy to clipboard"
