Add ui.explore settings to control view of explore pages (2) (#14094)

This is an alternative PR to #13687.

Add `[ui.explore]` settings to allow restricting the
explore pages to logged in users only and to disable the users explore page.

The two proposed settings are:

- `REQUIRE_SIGNIN_VIEW`: Only allows access to the explore pages if the
user is signed in. Also restricts
  - `/api/v1/user/search`
  - `/api/v1/users/{username}`
  - `/api/v1/users/{username}/repos`
  - but does not restrict `/api/v1/users/{username}/heatmap`
- `DISABLE_USERS_PAGE`: Disables the /explore/users page

Fix #2908

Close #13687 

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>

docs/content/doc/advanced/config-cheat-sheet.zh-cn.md

@@ -135,6 +135,11 @@ menu:
 - `ENABLE_REVERSE_PROXY_AUTO_REGISTRATION`: 允许通过反向认证做自动注册。
 - `ENABLE_CAPTCHA`: 注册时使用图片验证码。
 
+### Service - Expore (`service.explore`)
+
+- `REQUIRE_SIGNIN_VIEW`: **false**: 仅允许已登录的用户查看探索页面。
+- `DISABLE_USERS_PAGE`: **false**: 不显示用户探索页面。
+
 ## Webhook (`webhook`)
 
 - `QUEUE_LENGTH`: 说明: Hook 任务队列长度。