Fix various bugs for "install" page (#23194)

## TLDR

* Fix the broken page / broken image problem when click "Install"
* Close #20089
* Fix the Password Hash Algorithm display problem for #22942
* Close #23183
* Close #23184

## Details

### The broken page / broken image problem when click "Install"
(Redirect failed after install gitea #23184)

Before: when click "install", all new requests will fail, because the
server has been restarted. Users just see a broken page with broken
images, sometimes the server is not ready but the user would have been
redirect to "/user/login" page, then the users see a new broken page
(connection refused or something wrong ...)

After: only check InstallLock=true for necessary handlers, and sleep for
a while before restarting the server, then the browser has enough time
to load the "post-install" page. And there is a script to check whether
"/user/login" is ready, the user will only be redirected to the login
page when the server is ready.

### During new instance setup make 'Gitea Base URL' filled from
window.location.origin #20089

If the "app_url" input contains `localhost` (the default value from
config), use current window's location href as the `app_url` (aka
ROOT_URL)

### Fix the Password Hash Algorithm display problem for "Provide the
ability to set password hash algorithm parameters #22942"

Before: the UI shows `pbkdf2$50000$50`

<details>

![image](https://user-images.githubusercontent.com/2114189/221917143-e1e54798-1698-4fee-a18d-00c48081fc39.png)

</details>

After: the UI shows `pbkdf2`

<details>

![image](https://user-images.githubusercontent.com/2114189/221916999-97a15be8-2ebb-4a01-bf93-dac18e354fcc.png)

</details>

### GET data: net::ERR_INVALID_URL #23183

Cause by empty `data:` in `<link rel="manifest"
href="data:{{.ManifestData}}">`

---------

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

routers/install/routes.go

@@ -6,6 +6,7 @@ package install
 import (
 	goctx "context"
 	"fmt"
+	"html"
 	"net/http"
 	"path"
 
@@ -37,7 +38,7 @@ func installRecovery(ctx goctx.Context) func(next http.Handler) http.Handler {
 				// Why we need this? The first recover will try to render a beautiful
 				// error page for user, but the process can still panic again, then
 				// we have to just recover twice and send a simple error page that
-				// should not panic any more.
+				// should not panic anymore.
 				defer func() {
 					if err := recover(); err != nil {
 						combinedErr := fmt.Sprintf("PANIC: %v\n%s", err, log.Stack(2))
@@ -107,8 +108,9 @@ func Routes(ctx goctx.Context) *web.Route {
 
 	r.Use(installRecovery(ctx))
 	r.Use(Init(ctx))
-	r.Get("/", Install)
+	r.Get("/", Install) // it must be on the root, because the "install.js" use the window.location to replace the "localhost" AppURL
 	r.Post("/", web.Bind(forms.InstallForm{}), SubmitInstall)
+	r.Get("/post-install", InstallDone)
 	r.Get("/api/healthz", healthcheck.Check)
 
 	r.NotFound(web.Wrap(installNotFound))
@@ -116,5 +118,10 @@ func Routes(ctx goctx.Context) *web.Route {
 }
 
 func installNotFound(w http.ResponseWriter, req *http.Request) {
-	http.Redirect(w, req, setting.AppURL, http.StatusFound)
+	w.Header().Add("Content-Type", "text/html; charset=utf-8")
+	w.Header().Add("Refresh", fmt.Sprintf("1; url=%s", setting.AppSubURL+"/"))
+	// do not use 30x status, because the "post-install" page needs to use 404/200 to detect if Gitea has been installed.
+	// the fetch API could follow 30x requests to the page with 200 status.
+	w.WriteHeader(http.StatusNotFound)
+	_, _ = fmt.Fprintf(w, `Not Found. <a href="%s">Go to default page</a>.`, html.EscapeString(setting.AppSubURL+"/"))
 }