Allow programming groups with more than two people

[kiragrammel]

For my master's thesis, I limited programming groups to two people. The following changes need to be made so that more than two people can be in a programming group:

• In the ProgrammingGroup model remove validation max_group_size
• In views/programming_groups/_form.html.slim show the help-block which explains how to enter multiple IDs
• Adapt the texts on the views/programming_groups/new.html.slim page so that it is clear that you can have a group with more than two people e.g. replace programming pair with programming group
• Maybe also change the information in views/programming_groups/_info_pair_programming.html.slim
• When clicking the button Find Programming Partner only groups of two people are formed
  • Perhaps this behavior should remain this way, because otherwise the users have to wait even longer until more than one other person has been found.
  • If larger groups should be formed the method match_waiting_users in pg_matching_channel.rb needs to be adapted.
• In the editor statusbar currently only the status of one other user is shown this should be adapted so that you can see the status from all team members (Further changes for pair programming #1866 (comment)), Issue: Pair Programming: Improve connection information #1915
• On the programming groups index page currently all users from the group are listed in the table. If there are many users in a group you have to think about how to display them nicely (Create index, destroy, show, edit and update route for programming groups #1906 (comment)).
• If we have different group sizes one could add a size filter for the index table (Create index, destroy, show, edit and update route for programming groups #1906 (comment))

In the commit 97764bd some changes to limit the programming group to two people were made.