Switch to zulip-js

[arpith]

Currently the blocker is that graceful-fs and fs-readfile-promise need to be ignored.

In webpack, this was achieved by using externals

According to this links, parcel should support this in package.json / browser
parcel-bundler/parcel#200
https://github.com/defunctzombie/package-browser-field-spec

The other alternative is to ignore the files using babel alias (?!)
parcel-bundler/parcel#144

[arpith]

fs-readfile-promise is ignored in zulip/zulip-js@e3f7c4b

5de3ddd switches to zulip/zulip-js (git install)

presumably, 2.0.3 will be the npm version that can be switched to