Feat: allow users to login with phone number

Closes #945

This MR is only working on allowing users to sign in with their phone numbers since there is a serial of issues for adding the login with phone number feature.

The find_for_database_authentication has been overrode in ee/models/user.rb to enable signing in with email and username, so we'd better to put our code at there either. To enable unfinished code to be deployed, a feature flag should be added.

Proposal

1. Add a feature flag named phone_authenticatable
2. Override method find_for_database_authentication in user.rb

How to test

1. Create a new account with phone number provided
2. Sign in with phone number
3. You should sign in successfully