restructure: move bin/ and tests/ to src/

Also add symlinks from the old `bin/` and `tests/` locations to avoid
breaking scripts and other tools.

Motivations:

  * Scripts and tests no longer have to do dubious things like:

        require('ep_etherpad-lite/node_modules/foo')

    to access packages installed as dependencies in
    `src/package.json`.

  * Plugins can access the backend test helper library in a non-hacky
    way:

        require('ep_etherpad-lite/tests/backend/common')

  * We can delete the top-level `package.json` without breaking our
    ability to lint the files in `bin/` and `tests/`.

    Deleting the top-level `package.json` has downsides: It will cause
    `npm` to print warnings whenever plugins are installed, npm will
    no longer be able to enforce a plugin's peer dependency on
    ep_etherpad-lite, and npm will keep deleting the
    `node_modules/ep_etherpad-lite` symlink that points to `../src`.

    But there are significant upsides to deleting the top-level
    `package.json`: It will drastically speed up plugin installation
    because `npm` doesn't have to recursively walk the dependencies in
    `src/package.json`. Also, deleting the top-level `package.json`
    avoids npm's horrible dependency hoisting behavior (where it moves
    stuff from `src/node_modules/` to the top-level `node_modules/`
    directory). Dependency hoisting causes numerous mysterious
    problems such as silent failures in `npm outdated` and `npm
    update`. Dependency hoisting also breaks plugins that do:

        require('ep_etherpad-lite/node_modules/foo')

src/tests/container/specs/api/pad.js

@@ -0,0 +1,38 @@
+/*
+ * ACHTUNG: this file was copied & modified from the analogous
+ * <basedir>/tests/backend/specs/api/pad.js
+ *
+ * TODO: unify those two files, and merge in a single one.
+ */
+
+const settings = require('../../loadSettings').loadSettings();
+const supertest = require('supertest');
+
+const api = supertest(`http://${settings.ip}:${settings.port}`);
+const apiVersion = 1;
+
+describe('Connectivity', function () {
+  it('can connect', function (done) {
+    api.get('/api/')
+        .expect('Content-Type', /json/)
+        .expect(200, done);
+  });
+});
+
+describe('API Versioning', function () {
+  it('finds the version tag', function (done) {
+    api.get('/api/')
+        .expect((res) => {
+          if (!res.body.currentVersion) throw new Error('No version set in API');
+          return;
+        })
+        .expect(200, done);
+  });
+});
+
+describe('Permission', function () {
+  it('errors with invalid APIKey', function (done) {
+    api.get(`/api/${apiVersion}/createPad?apikey=wrong_password&padID=test`)
+        .expect(401, done);
+  });
+});