Move to pdf.js distributed with npm

pdf.js is available on npm. Importing the dependency from there instead
of vendoring it has certain benefits, e.g. the discoverability of
updates (especially security fixes).

app/assets/javascripts/pdf/index.vue

 <script>
-import pdfjsLib from 'vendor/pdf';
-import workerSrc from 'vendor/pdf.worker.min';
+import pdfjsLib from 'pdfjs-dist/build/pdf';
+import workerSrc from 'pdfjs-dist/build/pdf.worker.min';
 
 import page from './page/index.vue';
 

package.json

@@ -93,6 +93,7 @@
     "monaco-editor": "^0.15.6",
     "monaco-editor-webpack-plugin": "^1.7.0",
     "mousetrap": "^1.4.6",
+    "pdfjs-dist": "^2.0.943",
     "pikaday": "^1.6.1",
     "popper.js": "^1.14.7",
     "prismjs": "^1.6.0",

spec/javascripts/pdf/index_spec.js

 import Vue from 'vue';
-import { GlobalWorkerOptions } from 'vendor/pdf';
-import workerSrc from 'vendor/pdf.worker.min';
+import { GlobalWorkerOptions } from 'pdfjs-dist/build/pdf';
+import workerSrc from 'pdfjs-dist/build/pdf.worker.min';
 
 import PDFLab from '~/pdf/index.vue';
 import { FIXTURES_PATH } from 'spec/test_constants';

spec/javascripts/pdf/page_spec.js

 import Vue from 'vue';
-import pdfjsLib from 'vendor/pdf';
-import workerSrc from 'vendor/pdf.worker.min';
+import pdfjsLib from 'pdfjs-dist/build/pdf';
+import workerSrc from 'pdfjs-dist/build/pdf.worker.min';
 
 import PageComponent from '~/pdf/page/index.vue';
 import mountComponent from 'spec/helpers/vue_mount_component_helper';

yarn.lock

@@ -7362,6 +7362,11 @@ nice-try@^1.0.4:
   resolved "https://registry.yarnpkg.com/nice-try/-/nice-try-1.0.4.tgz#d93962f6c52f2c1558c0fbda6d512819f1efe1c4"
   integrity sha512-2NpiFHqC87y/zFke0fC0spBXL3bBsoh/p5H1EFhshxjCR5+0g2d6BiXbUFz9v1sAcxsk2htp2eQnNIci2dIYcA==
 
+node-ensure@^0.0.0:
+  version "0.0.0"
+  resolved "https://registry.yarnpkg.com/node-ensure/-/node-ensure-0.0.0.tgz#ecae764150de99861ec5c810fd5d096b183932a7"
+  integrity sha1-7K52QVDemYYexcgQ/V0Jaxg5Mqc=
+
 node-fetch@1.6.3:
   version "1.6.3"
   resolved "https://registry.yarnpkg.com/node-fetch/-/node-fetch-1.6.3.tgz#dc234edd6489982d58e8f0db4f695029abcd8c04"
@@ -8104,6 +8109,14 @@ pbkdf2@^3.0.3:
     safe-buffer "^5.0.1"
     sha.js "^2.4.8"
 
+pdfjs-dist@^2.0.943:
+  version "2.0.943"
+  resolved "https://registry.yarnpkg.com/pdfjs-dist/-/pdfjs-dist-2.0.943.tgz#32fb9a2d863df5a1d89521a0b3cd900c16e7edde"
+  integrity sha512-iLhNcm4XceTHRaSU5o22ZGCm4YpuW5+rf4+BJFH/feBhMQLbCGBry+Jet8Q419QDI4qgARaIQzXuiNrsNWS8Yw==
+  dependencies:
+    node-ensure "^0.0.0"
+    worker-loader "^2.0.0"
+
 performance-now@^2.1.0:
   version "2.1.0"
   resolved "https://registry.yarnpkg.com/performance-now/-/performance-now-2.1.0.tgz#6309f4e0e5fa913ec1c69307ae364b4b377c9e7b"