fix(middleware): set baseURL to external host if provided

[twotobinary]

Added a fix for issue with reverse proxy: #368

To test this, I added external_host: https://my.domain.com to the config yml and created a docker image. Paths no longer contain the internal address that is inaccessible by the client.

[WithoutPants]

Code looks fine.

[bnkai]

Looks ok to me also.
@twotobinary did you test also the ENV part ? Instead of changing the config, just add the appropriate STASH_EXTERNAL_HOST env to the compose file for your docker test (should be ok though)

[twotobinary]

@bnkai I just finished testing by adding env variable: STASH_EXTERNAL_HOST (and removed from config) and it works as expected. I also did a quick test without the variable to make sure original behavior was restored.