Replace static fixture by generated one in issue_spec.js

What does this MR do?

• clean up issue_spec.js
• introduce an alternative approach to #19445 (closed)
  • rename rake teaspoon to rake teaspoon:tests
  • introduce rake teaspoon:fixtures which generates fixtures using RSpec
  • introduce rake teaspoon which runs rake teaspoon:fixtures and rake teaspoon:tests

Why was this MR needed?

• many duplications
• missing existence checks
• missing conditions
• static fixtures don't match real views

Reasoning

I want to explain some of my decisions here, so that they stay visible for future discussions.

Why not HAML?

• same number of HAML templates as number of fixtures (many input files)
• embedded logic less readable
• can not be rendered by JavaScript (because of inline Ruby)

Why RSpec?

• real controllers for fixtures
• spys available for mocking
• easily report failed fixture generations

Why not magic_lamp? (#19445 (closed))

• introduces another dependency/tool
• needs to run a server concurrently to teaspoon
• makes it harder to use a JavaScript test runner
• static HTML files serve faster