sso_*: move sign_in and sign_out HTML pages to sso_proxy

[Jusshersmith]

Problem

The current setup of html sign in and sign out pages causes some extra friction with implementing https://github.com/buzzfeed/sso/pull/252 - this change gives us some extra flexibility here. Also, having sso_proxy first send requests to sso_auth, which subsequently renders the sign in page (and redirects back) results in extra requests and extra complexity (for example, an extra layer of nested redirects needs to be specified).

Solution

Move the sign_in and sign_out HTML pages to be rendered by sso_proxy instead of sso_auth. In doing so, simplify the flow in part between sso_proxy and sso_authenticator.

Notes

This is a big change, so I'm working to add some more detailed descriptions of the changes (and going over any TODO's) to aid in better understanding them, as well as reviewing the PR in general.

[codecov[bot]]

Codecov Report

Merging #261 into master will decrease coverage by 0.01%. The diff coverage is 65.9%.

@@            Coverage Diff            @@
##           master    #261      +/-   ##
=========================================
- Coverage   62.51%   62.5%   -0.02%     
=========================================
  Files          54      55       +1     
  Lines        4199    4211      +12     
=========================================
+ Hits         2625    2632       +7     
- Misses       1385    1386       +1     
- Partials      189     193       +4

Impacted Files	Coverage Δ
internal/proxy/providers/providers.go	0% <ø> (ø)	:arrow_up:
internal/proxy/providers/sso.go	69.14% <0%> (+0.53%)	:arrow_up:
internal/proxy/providers/test_provider.go	0% <0%> (ø)	:arrow_up:
...nternal/proxy/providers/singleflight_middleware.go	0% <0%> (ø)	:arrow_up:
internal/pkg/templates/templates.go	100% <100%> (ø)	:arrow_up:
internal/proxy/static_files.go	73.33% <73.33%> (ø)
internal/auth/authenticator.go	83.85% <84.21%> (-2.33%)	:arrow_down:
internal/proxy/oauthproxy.go	59.15% <86.27%> (+4.71%)	:arrow_up:

[Jusshersmith]

Marked as ready for review, but see Notes in description.