Modify User model based on feedback

[devukurup]

Make modifications in #2 (PR #21) based on feedback.

• [x] Place before_save above validations. so that redundant email becomes invalid before saving. Add test cases to ensure the same. Ref : Comment
• [x] Replace email attribute line in create_users migration file to avoid explicit coding to create the index. Ref : Comment
• [x] Remove redundant tests. Ref : Comment
• [x] Test validity of first_name and last_name for maximum length 50 instead of 49. Ref : Comment
• [x] In the test for lowercase email validation, use @user.update! method. Ref : Comment