Support /terminal being hosted deeper than the web server root level

[gjsjohnmurray]

This PR fixes #142, allowing WebTerminal to work on sites that configure a single web server to host apps for multiple InterSystems servers.

It also updates gulp to version 4 so it works with modern Node versions.

[nikitaeverywhere]

Hello John! Thanks so much for your work. I'll have a chance to review it over this weekend – will let you know.

Let me know if that's OK

[gjsjohnmurray]

I'll have a chance to review it over this weekend – will let you know.

That'll be fine.

@ZitRos FYI I'm also thinking of doing another PR so my VS Code extension can work without the user having to edit the WebTerminal.Router class.

[nikitaeverywhere]

@ZitRos FYI I'm also thinking of doing another PR so my VS Code extension can work without the user having to edit the WebTerminal.Router class.

I'm supportive. At the time when development started, we didn't have VisualStudio code yet which I love (i.e. in general, a pipeline to work with external editors)

[nikitaeverywhere]

@gjsjohnmurray thanks a ton for the PR. I currently don't have an ability to validate the release, please do it yourself (try to install the old version and update from it through the UI as well).

[nikitaeverywhere]

The code looks good!

[gjsjohnmurray]

@ZitRos thanks for merging and releasing this. I confirmed that UI upgrade from 4.9.3 to 4.9.4 worked.