Add server info to response header

[zzgvh]

For debugging purposes the Up app (and possibly other clients too) could use the RSR version info, like the one that's visible in the admin.

Suggested implementation of this is as a custom response header using the same format as in the admin:

X-RSR-Version: <version tag> (<commit hash>) [<branch name>]

A small middleware class is one way to implement this across all server responses. But I'd like to hear from @carlio or @osgl if this could be added at an even "lower" level in the webserver configuration.

[carlio]

Given that the version number is an artefact of the code and not the provisioning environment, I don't think it makes sense to add it to the nginx config. Apart from anything else, you would have to update nginx at the same time as RSR, while if it is done in RSR, the environment does not need to know or care.

[zzgvh]

Ok, middleware it is then!

[kardan]

What's the status on this?

[KasperBrandt]

Test plan

GIVEN a request to RSR
WHEN a response is given
THEN the response header in the form of X-RSR-Version: tag={tag}, commit={commit}, branch={branch} is returned