Fix/remove user/soft removal

A brief description of the purpose of the changes contained in this PR.

Fixes some small issues seen after deploying the delete user feature.

It should fix the following problem:

• email formatting
• setting is_active to False on user deletion
• hide deleted user on competition participant list + add an option to display them

Related PR

#1691 (original) #1716 (for deployment)

Issues this PR resolves

#1159

Reminder on the hand testing checklist

• Create a new user
• Complete its profile with as much information as possible
• Create an organization
• Create a competition
• Create a Queue
• Add a submission
• Make the competition and the submission public
• Log out and log in with another user
• Take a look at the new user profile
• Log out and log in with the new user's account
• Delete the account using the account view
• Check your email (or the message in the console)
• Click on the given link
• Check that admins received an email with information on the user's deletion
• Check the the deleted user's got a confirmation email
• Try to log in with the deleted user's account. And fail
• Log in as another user
• Check that the competition and submissions of the removed user are still here but without personal data
• Take a look at the removed user's profile and check that no personal information is displayed

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge