feature: support running the app under a base URL

**Is your feature request related to a problem? Please describe.**

Related issue: #3356

I would like to host REMS under a non-root URL e.g. `https://my.domain/app1/` so that I may use path-based routing to gather related services under the same domain. This greatly simplifies sso and other shared origin requirements.

My reading and experiments lead me to think that currently I can't do this because REMS is a single-page app that doesn't support a non-base URL configuration. I have tried reverse proxy config in front but it is not sufficient because using e.g. `nginx` `sub_filter` or Apache `mod_proxy_html` can only rewrite paths that appear explicitly in responses and not the application paths dynamically generated in script.

**Describe the solution you'd like**

I would like a configuration point `base-url` that defaults to `/` but can be changed to something else e.g. `/app1/`, so that the REMS application paths are all relative to that base URL.

**Describe alternatives you've considered**

I have tried a reverse proxy. The best I can get REMS to work with is a proxy to a different port and/or subdomain, but not a new base path.

**Additional context**
Thanks for creating and maintaining REMS. It's a great product.