Easier setup for GitLab pages by autogenerating .gitlab-ci.yml for any static site generator

Currently, to set up GitLab Pages the user needs to write a .gitlab-ci.yml like:

pages:
  script:
  - mkdir .public
  - cp -r * .public
  - mv .public public
  artifacts:
    paths:
    - public
  only:
  - master

or

image: ruby:2.1

pages:
  script:
  - gem install jekyll
  - jekyll build -d public/
  artifacts:
    paths:
    - public
  only:
    - master

Asking the user to write a relatively long file that they don't fully understand makes GitLab Pages feel more complex than it needs to.

Adding a "Enable GitLab Pages"button the UI that generates this for them based on a simple (autodetected?) choice between "Plain HTML", "Jekyll", etc. would be much better from a workflow perspective.

cc @ayufan @JobV @sytses