1. `page.getViewpoint()` now takes in JSON instead of an integer.
2. Promises are returned from loading and rendering tasks.

See https://mozilla.github.io/pdf.js/examples/index.html for more
details.

Make pdf.js render CJK characters

Closes #62152

See merge request gitlab-org/gitlab-ce!31220

Instead of complicated splicing, we can simply return all pages.
Promise.all will take care of the correct ordering for us.

There was an implicit assumption that the pages returned from the
Promise of `pdf.getPage(num)` would return in order, but no such
guarantee exists. To handle this, we explicitly set which array index
based on the page number and then trigger a Vue update via `splice`.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64467

As mentioned in
https://github.com/wojtekmaj/react-pdf/blob/master/README.md,
pdf.js needs the bundled cMaps files to work.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/62152

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).