perf: access to admin page changed to template rendering (#2259)

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#### What type of PR is this?

/kind improvement

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#### What this PR does / why we need it:

访问后台管理时使用重定向，在有反向代理且配置不完善的情况下，会导致重定向的域名发生错误。

https://spring.io/guides/gs/serving-web-content/
#### Which issue(s) this PR fixes:

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Fixes #2254

#### Special notes for your reviewer:
/cc @halo-dev/sig-halo 

#### Does this PR introduce a user-facing change?



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```release-note
修改后台管理页面为直接渲染而不是重定向，以更好的兼容反向代理
```

src/main/java/run/halo/app/controller/content/MainController.java

@@ -50,11 +50,9 @@ public class MainController {
     }
 
     @GetMapping("${halo.admin-path:admin}")
-    public void admin(HttpServletResponse response) throws IOException {
-        String adminIndexRedirectUri =
-            HaloUtils.ensureBoth(haloProperties.getAdminPath(), HaloUtils.URL_SEPARATOR)
-                + INDEX_REDIRECT_URI;
-        response.sendRedirect(adminIndexRedirectUri);
+    public String admin() throws IOException {
+        return HaloUtils.ensureBoth(haloProperties.getAdminPath(), HaloUtils.URL_SEPARATOR)
+            + INDEX_REDIRECT_URI;
     }
 
     @GetMapping("version")
@@ -64,14 +62,12 @@ public class MainController {
     }
 
     @GetMapping("install")
-    public void installation(HttpServletResponse response) throws IOException {
+    public String installation() throws IOException {
         boolean isInstalled = optionService
             .getByPropertyOrDefault(PrimaryProperties.IS_INSTALLED, Boolean.class, false);
         if (!isInstalled) {
-            String installRedirectUri =
-                StringUtils.appendIfMissing(this.haloProperties.getAdminPath(), "/")
-                    + INSTALL_REDIRECT_URI;
-            response.sendRedirect(installRedirectUri);
+            return StringUtils.appendIfMissing(this.haloProperties.getAdminPath(), "/")
+                + INSTALL_REDIRECT_URI;
         } else {
             throw new NotFoundException("404");
         }
@@ -103,4 +99,4 @@ public class MainController {
             response.sendRedirect(HaloUtils.normalizeUrl(favicon));
         }
     }
-}
+}