Add context retrieval for reads via books-mf2

Using the service books-mf2, we can retrieve the context for read data.

As it's a Heroku-based app, there's the risk that this can timeout, so
we need to increase the timeouts available for context retrieval events.

src/events/fetch-context/books-mf2.js

@@ -0,0 +1,31 @@
+const fetch = require('node-fetch')
+const logger = require('@architect/shared/logger')
+
+function name () {
+  return 'Books-MF2'
+}
+
+function isBooksMf2Url (url) {
+  return (url.indexOf('https://books-mf2.herokuapp.com/') > -1)
+}
+
+async function fetchContext (url) {
+  if (!isBooksMf2Url(url)) {
+    return
+  }
+  const response = await fetch(url)
+  if (!response.ok) {
+    const text = await response.text()
+    logger.warn('Failed to fetch context from Books-MF2', `${url}\n${text}`)
+    return
+  }
+  const mf2 = await response.json()
+  if (!('items' in mf2) || !mf2.items.length) return
+  return mf2.items[0].properties
+}
+
+module.exports = {
+  name,
+  isBooksMf2Url,
+  fetchContext
+}

src/events/fetch-context/config.arc

@@ -0,0 +1,2 @@
+@aws
+timeout 15

src/events/fetch-context/index.js

@@ -1,5 +1,6 @@
 const arc = require('@architect/functions')
 const logger = require('@architect/shared/logger')
+const booksMf2 = require('./books-mf2')
 const eventbrite = require('./eventbrite')
 const granary = require('./granary')
 const meetup = require('./meetup')
@@ -10,6 +11,8 @@ async function getHandler (url) {
     return meetup
   } else if (eventbrite.isEventbriteUrl(url)) {
     return eventbrite
+  } else if (booksMf2.isBooksMf2Url(url)) {
+    return booksMf2
   } else {
     return granary
   }