Limit the number of notifications requests

[jorg-vr]

This pull request limits the numbers of notifications requests in two ways;

1. Notifications are only updated automatically if a user has open questions
2. Notifications get updated less often if a tab is inactive

A new gem counter_culture was added that introduces conditional counter cache. This is used to cache the number of open questions for a user.

This should be run manually in the console, to initialize counter cache:

User.find_each do |u|
   u.update(open_questions_count: u.questions.where.not(question_state: :answered).count)
 end

• Tests were added

Closes #3576

[bmesuere]

This is a lot more readable! 👍