Add backend implementation to trigger the maintenance mode

**Is your feature request related to a problem? Please describe.**
Add backend implementation to trigger the maintenance mode

**Describe the solution you'd like**
When maintenance mode will be triggered then only the old registered users will able to log in but the new users who will try to register will not be able to login whereas after the successful login of old registered user they will get the message in a popup on the client side like "**The site is kept under maintenance, please keep patience and try later!!**"

**Additional context**
**Working on it!**