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    problems such as silent failures in `npm outdated` and `npm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/bin/doc/package-lock.json

@@ -0,0 +1,13 @@
+{
+  "name": "node-doc-generator",
+  "version": "0.0.0",
+  "lockfileVersion": 1,
+  "requires": true,
+  "dependencies": {
+    "marked": {
+      "version": "0.8.2",
+      "resolved": "https://registry.npmjs.org/marked/-/marked-0.8.2.tgz",
+      "integrity": "sha512-EGwzEeCcLniFX51DhTpmTom+dSA/MG/OBUDjnWtHbEnjAH180VzUeAw+oE4+Zv+CoYBWyRlYOTR0N8SO9R1PVw=="
+    }
+  }
+}