Replace marked/turndown/purify with ribbit editor

This commit moves all html/markdown editor logic to the ribbit library.

Submodule tracks https://git.evilchi.li/evilchili/ribbit.git.
The dist files in static/ are committed copies; the submodule
provides source access for development.

We use pre_format and pre_build slam hooks to ensure the submodules are
initialized and updated correctly, and to perform the dist file copy.

.slam/hooks/pre_build

@@ -0,0 +1,18 @@
+#!/bin/sh
+set -e
+
+RIBBIT_DIR="lib/ribbit"
+STATIC_DIR="src/ttfrog/themes/default/static"
+
+echo "Building ribbit..."
+cd "$RIBBIT_DIR"
+npm install --silent
+npm run build --silent
+cd - > /dev/null
+
+echo "Copying ribbit dist files..."
+cp "$RIBBIT_DIR/dist/ribbit.js" "$STATIC_DIR/"
+cp "$RIBBIT_DIR/dist/ribbit.min.js" "$STATIC_DIR/"
+cp "$RIBBIT_DIR/src/ribbit.css" "$STATIC_DIR/"
+
+echo "ribbit build complete."