title: Calculate amount of users who was active two days in a row mysql
body: <p>I need to calculate amount of users who was active two days in a row.
  I have table 'activity'</p>
  
  <p>user_id | login_time </p>
  
  <p>1234    |  2019-04-15</p>
  
  <p>1456    |  2019-04-15</p>
  
  <p>1234    |  2019-04-16</p>
  
  <p>1456    |  2019-04-17</p>
  
  <p>How I can calculate amount of users who was active on 15th and 16th too. </p>