Freeze tutorials: fixes and cleanup

- Move JS code samples to JS folder (future-proofing for other
  languages)
- Make "Freeze a Trust Line" tutorial interactive
- Add necessary functionality to interactive_tutorial helper JS
- Rework freeze a trust line tutorial/code sample to make an incoming
  line from another address and make looking up trust lines a
  full-fledged step

content/_code-samples/freeze/js/check-individual-freeze.js

@@ -0,0 +1,54 @@
+// Dependencies for Node.js.
+// In browsers, use <script> tags as in the example demo.html.
+if (typeof module !== "undefined") {
+  // gotta use var here because const/let are block-scoped to the if statement.
+  var xrpl = require('xrpl')
+}
+
+// Connect -------------------------------------------------------------------
+async function main() {
+  console.log("Connecting to Mainnet...")
+  const client = new xrpl.Client('wss://s1.ripple.com')
+  await client.connect()
+
+  const my_address = 'rf1BiGeXwwQoi8Z2ueFYTEXSwuJYfV2Jpn'
+  const counterparty_address = 'rUpy3eEg8rqjqfUoLeBnZkscbKbFsKXC3v'
+  const frozen_currency = 'USD'
+
+  // Look up current state of the trust line ----------------------------------
+  const account_lines = {
+    command: 'account_lines',
+    account: my_address,
+    peer: counterparty_address,
+  }
+
+  console.log(`Looking up all trust lines from
+              ${counterparty_address} to ${my_address}`)
+
+  const data = await client.request(account_lines)
+  
+  // Find the trust line for our frozen_currency ------------------------------
+  let trustline = null
+  for (let i = 0; i < data.result.lines.length; i++) {
+    if(data.result.lines[i].currency === frozen_currency) {
+      trustline = data.result.lines[i]
+      break
+    }
+  }
+
+  if(trustline === null) {
+    throw `There was no ${frozen_currency} trust line`
+  }
+
+  // Check if the trust line is frozen -----------------------------------------
+  console.log('Trust line frozen from our side?',
+              trustline.freeze === true)
+  console.log('Trust line frozen from counterparty\'s side?',
+              trustline.freeze_peer === true)
+  
+  await client.disconnect()
+
+  // End main()
+}
+
+main().catch(console.error)