make a /boot/GRIST_BOOT_KEY page for diagnosing configuration problems (#850)

This is a start at a page for diagnosing problems while setting up Grist. Starting to add some diagnostics based on feedback in github issues. We should make Grist installation easier! But when there is a problem it should be easier to diagnose than it is now, and this may help. The page is ugly and doesn't have many diagnostics yet, but we can iterate.

Visit `/boot` on a Grist server for tips on how to use this feature.

test/nbrowser/Boot.ts

@@ -0,0 +1,52 @@
+import {assert, driver} from 'mocha-webdriver';
+import * as gu from 'test/nbrowser/gristUtils';
+import {server, setupTestSuite} from 'test/nbrowser/testUtils';
+import * as testUtils from 'test/server/testUtils';
+
+describe('Boot', function() {
+  this.timeout(30000);
+  setupTestSuite();
+
+  let oldEnv: testUtils.EnvironmentSnapshot;
+
+  afterEach(() => gu.checkForErrors());
+
+  async function hasPrompt() {
+    assert.include(
+      await driver.findContentWait('p', /diagnostics page/, 2000).getText(),
+      'A diagnostics page can be made available');
+  }
+
+  it('gives prompt about how to enable boot page', async function() {
+    await driver.get(`${server.getHost()}/boot`);
+    await hasPrompt();
+  });
+
+  describe('with a GRIST_BOOT_KEY', function() {
+    before(async function() {
+      oldEnv = new testUtils.EnvironmentSnapshot();
+      process.env.GRIST_BOOT_KEY = 'lala';
+      await server.restart();
+    });
+
+    after(async function() {
+      oldEnv.restore();
+      await server.restart();
+    });
+
+    it('gives prompt when key is missing', async function() {
+      await driver.get(`${server.getHost()}/boot`);
+      await hasPrompt();
+    });
+
+    it('gives prompt when key is wrong', async function() {
+      await driver.get(`${server.getHost()}/boot/bilbo`);
+      await hasPrompt();
+    });
+
+    it('gives page when key is right', async function() {
+      await driver.get(`${server.getHost()}/boot/lala`);
+      await driver.findContentWait('h2', /Grist is reachable/, 2000);
+    });
+  });
+});