Create migration to erase body in read notices.

app/scripts/migrations/012.js

@@ -0,0 +1,36 @@
+const version = 12
+
+/*
+
+This migration modifies our notices to delete their body after being read.
+
+*/
+
+const clone = require('clone')
+
+module.exports = {
+  version,
+
+  migrate: function (originalVersionedData) {
+    let versionedData = clone(originalVersionedData)
+    versionedData.meta.version = version
+    try {
+      const state = versionedData.data
+      const newState = transformState(state)
+      versionedData.data = newState
+    } catch (err) {
+      console.warn(`MetaMask Migration #${version}` + err.stack)
+    }
+    return Promise.resolve(versionedData)
+  },
+}
+
+function transformState (state) {
+  const newState = state
+  newState.NoticeController.noticesList.forEach((notice) => {
+    if (notice.read) {
+      notice.body = ''
+    }
+  })
+  return newState
+}