Feat: Add user registration toggle feature (#6327)

### What problem does this PR solve?

Feat: Add user registration toggle feature. Added a user registration
toggle REGISTER_ENABLED in the settings and .env config file. The user
creation interface now checks the state of this toggle to control the
enabling and disabling of the user registration feature.

the front-end implementation is done, the registration button does not
appear if registration is not allowed. I did the actual tests on my
local server and it worked smoothly.
### Type of change

- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: wenju.li <wenju.li@deepctr.cn>
Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

docker/.env

@@ -146,3 +146,6 @@ TIMEZONE='Asia/Shanghai'
 # ENDPOINT=http://oss-cn-hangzhou.aliyuncs.com
 # REGION=cn-hangzhou
 # BUCKET=ragflow65536
+
+# user registration switch
+REGISTER_ENABLED=1