Use find_by instead of where + first

What does this MR do?

Update usages of where.first and change them to use find_by instead.

# bad
User.where(name: 'Bruce').first
User.where(name: 'Bruce').take

# good
User.find_by(name: 'Bruce')

I've tried to enforce this automatically with Rails/FindBy in the .rubocop file which is supposed to be enabled by default but doesn't work at all.

References:

• https://github.com/rubocop-hq/rails-style-guide#find_by
• https://docs.rubocop.org/projects/rails/en/stable/cops_rails/#railsfindby

I've modified this with the help of Atom:

Closes #211877 (closed)