fix: Redirect to '?redirect' query parameter after successful login

[bryphe-coder]

Fixes #304
Unblocks #298

After logging in, the login flow should redirect to a whatever path is specified by the ?redirect query parameter. This is important for cases like #298 - where we need to set ?redirect=%2Fcli_auth, but also really any case where the user is linked and might have to go back to the login screen.

The fix is simple - just check if the redirect query parameter is set, and if it is, use that as the path to redirect to on success. Also adds a test case - we had one checking that we redirect to the default (root /) url, but not one of the ?redirect param

[codecov]

Codecov Report

Merging #307 (99054f9) into main (4954a6b) will increase coverage by 0.07%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main     #307      +/-   ##
==========================================
+ Coverage   67.95%   68.02%   +0.07%     
==========================================
  Files         135      135              
  Lines        7149     7156       +7     
  Branches       73       74       +1     
==========================================
+ Hits         4858     4868      +10     
+ Misses       1804     1800       -4     
- Partials      487      488       +1     

Flag	Coverage Δ
unittest-go-macos-latest	66.36% <ø> (+0.21%)	⬆️
unittest-go-ubuntu-latest	67.59% <ø> (+0.32%)	⬆️
unittest-go-windows-latest	65.98% <ø> (-0.06%)	⬇️
unittest-js	64.81% <88.88%> (+0.19%)	⬆️

Impacted Files	Coverage Δ
site/components/SignIn/SignInForm.tsx	96.87% <88.88%> (-3.13%)	⬇️
provisioner/echo/serve.go	47.36% <0.00%> (-3.16%)	⬇️
peer/conn.go	79.48% <0.00%> (-1.29%)	⬇️
coderd/provisionerdaemons.go	57.84% <0.00%> (ø)
peer/channel.go	87.19% <0.00%> (+2.43%)	⬆️
peerbroker/dial.go	85.71% <0.00%> (+9.52%)	⬆️

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