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    problems such as silent failures in `npm outdated` and `npm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')

bin/plugins/lib/backend-tests.yml

@@ -1,51 +0,0 @@
-# You need to change lines 38 and 46 in case the plugin's name on npmjs.com is different
-# from the repository name
-
-name: "Backend tests"
-
-# any branch is useful for testing before a PR is submitted
-on: [push, pull_request]
-
-jobs:
-  withplugins:
-    # run on pushes to any branch
-    # run on PRs from external forks
-    if: |
-         (github.event_name != 'pull_request')
-         || (github.event.pull_request.head.repo.id != github.event.pull_request.base.repo.id)
-    name: with Plugins
-    runs-on: ubuntu-latest
-
-    steps:
-    - name: Install libreoffice
-      run: |
-        sudo add-apt-repository -y ppa:libreoffice/ppa
-        sudo apt update
-        sudo apt install -y --no-install-recommends libreoffice libreoffice-pdfimport
-
-    # clone etherpad-lite
-    - name: Install etherpad core
-      uses: actions/checkout@v2
-      with:
-        repository: ether/etherpad-lite
-
-    - name: Install all dependencies and symlink for ep_etherpad-lite
-      run: bin/installDeps.sh
-
-    # clone this repository into node_modules/ep_plugin-name
-    - name: Checkout plugin repository
-      uses: actions/checkout@v2
-      with:
-        path: ./node_modules/${{github.event.repository.name}}
-
-    - name: Install plugin dependencies
-      run: |
-        cd node_modules/${{github.event.repository.name}}
-        npm ci
-
-    # configures some settings and runs npm run test
-    - name: Run the backend tests
-      run: tests/frontend/travis/runnerBackend.sh
-
-##ETHERPAD_NPM_V=1
-## NPM configuration automatically created using bin/plugins/updateAllPluginsScript.sh