fix: to support --rootpath

[mayzhang2000]

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Optional. My organization is added to USERS.md.
• I've signed the CLA and my build is green (troubleshooting builds).

[codecov]

Codecov Report

Merging #3503 into master will increase coverage by 0.02%.
The diff coverage is 23.80%.

@@            Coverage Diff             @@
##           master    #3503      +/-   ##
==========================================
+ Coverage   43.04%   43.07%   +0.02%     
==========================================
  Files         183      183              
  Lines       20266    20206      -60     
  Branches      237      273      +36     
==========================================
- Hits         8724     8703      -21     
+ Misses      10538    10497      -41     
- Partials     1004     1006       +2     

Impacted Files	Coverage Δ
server/server.go	55.17% <5.88%> (-0.85%)	⬇️
util/swagger/swagger.go	100.00% <100.00%> (ø)
server/application/application.go	28.11% <0.00%> (-0.21%)	⬇️
server/project/util.go	100.00% <0.00%> (ø)
controller/sync_phase.go	100.00% <0.00%> (ø)
ui/src/app/applications/components/utils.tsx	49.83% <0.00%> (ø)
ui/src/app/shared/services/projects-service.ts	19.71% <0.00%> (ø)

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[alexmt]

The server still redirects to root path. The changes have to be implemented in handler implementation. E.g.

		Handler: http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
			target := "https://" + req.Host
			if rootPath != "" {
				target += strings.TrimRight(strings.TrimLeft(rootPath, "/"), "/")
			}
			target += req.URL.Path
			if len(req.URL.RawQuery) > 0 {
				target += "?" + req.URL.RawQuery
			}
			http.Redirect(w, req, target, http.StatusTemporaryRedirect)
		}),

[alexmt]

I've tried to verify https->http redirect behavior and noticed a bug.

http://localhost:8080/argocd1/applications was redirected to https://localhost:8080argocd1/applications . Can you take a look please?

[alexmt]

The req.URL.Path starts with /, so we might get double slash at https://github.com/argoproj/argo-cd/pull/3503/files/fd61815dfb544c9227f3814b9d44783a277b501e..b4cbc25337e748139cea282975dd6ed3fbe486d0#diff-91bbeda7eb98a7adc57b9e47e2cf5c2bR684.

Suggested change

	target := "https://" + req.Host + "/"
	target := "https://" + req.Host

[alexmt]

LGTM