[Bug Report] BaseURL is set to reverse proxy IP, not web client's

[twotobinary]

Describe the bug
I'm using stash on my Kubernetes cluster with Istio mTLS enabled and Pomerium as my SSO provider which acts as a reverse proxy so this results in following access:

1. https://<external stash domain>
2. Istio routes to pomerium service within kubernetes cluster -> pomerium-proxy.namespace.svc.cluster.local:443
3. Pomerium routes to stash service -> stash.namespace.svc.cluster.local:9999
4. Pomerium sets baseURL to stash.namespace.svc.cluster.local:9999 when it should be my external stash domain.

To Reproduce
Access stash from a reverse proxy and the host will be improperly set at BaseURLMiddleware.

It would be nice if we can override this value with some configuration value such as external_host: https://external.host.com

Expected behavior
I expect to see my base URL set to the address my client is accessing stash from.

As a workaround, I can still stream all files as long as I manually set the domain to the external domain.

[WithoutPants]

Closed via #369