Support for external OIDC providers and implicit login flows

[jessesuen]

Resolves #671.

Major refactor of OAuth2/OIDC logic to support OAuth2 authentication in a more generic way. This change enables support for external IDPs without going through dex. We now can perform the implicit OAuth2 login flow, which is required when performing CLI logins directly to external IDPs, which do not have access to the OAuth2 client secret.

External OIDC is configured in the following manner in the argocd.secret:

apiVersion: v1
data:
  url: http://example.com
  oidc.config: |
    name: Okta
    issuer: https://dev-12345.oktapreview.com
    clientID: abc12345

NOTE: we still require some UI changes to create the login button if oidc.config is specified.

[codecov-io]

Codecov Report

Merging #727 into master will increase coverage by 0.31%.
The diff coverage is 7.02%.

@@            Coverage Diff             @@
##           master     #727      +/-   ##
==========================================
+ Coverage   22.37%   22.69%   +0.31%     
==========================================
  Files          40       40              
  Lines        6252     6174      -78     
==========================================
+ Hits         1399     1401       +2     
+ Misses       4666     4587      -79     
+ Partials      187      186       -1

Impacted Files	Coverage Δ
util/oidc/templates.go	0% <ø> (ø)
util/session/sessionmanager.go	27.41% <0%> (+4.6%)	⬆️
server/server.go	30.41% <0%> (-2.79%)	⬇️
util/oidc/oidc.go	2.76% <2.76%> (ø)
util/rand/rand.go	84.61% <84.61%> (ø)

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[alexmt]

LGTM