Add X-WEBAUTH-USER header

[cawfeecoder]

Adding the X-WEBAUTH-USER header to support enterprise applications where authentication happens at a proxy in front of the application using X509/PKI and the username is passed through via header.

[cawfeecoder]

@prymitive I'll fix this up tonight so it's currently failing test.

[prymitive]

What is the intention behind this? Is it just to pre-populate the author field or to force specific author in silences? Author field value is stored in localStorage so once you input something it will be remembered for future silences.
On top of that I have a few generic comments:

• There's a bunch of go modules changes that seem unrelated
• Using cookies to communicate between the backend and frontend is not the right way, there's an API for that
• There are more possible headers than X-WEBAUTH-USER that can be used for authentication, I don't think that adding code that deals specifically with that one header is the best way. Adding a config option to specify a header name and a way of extracting it (maybe via regex) seems like a more generic way.