Since we're prepending the ActiveRecord::Extension module, we
can take advantage of it and avoid using an alias to extend the
original #attributes method.

It's not needed anymore as we require `#cache_key` instead.

This commit also includes some changes in specs to use
`Class.new` approach.

This allows using `CacheMarkdownField` for models that are not backed
by ActiveRecord.

When the including class inherits `ActiveRecord::Base` we include
`Gitlab::MarkdownCache::ActiveRecord::Extension`. This will cause the
markdown fields to be rendered and the generated HTML stored in a
`<field>_html` attribute on the record. We also store the version
used for generating the markdown.

All other classes that include this model will include the
`Gitlab::MarkdownCache::Redis::Extension`. This add the `<field>_html`
attributes to that model and will generate the html in them. The
generated HTML will be cached in redis under the key
`markdown_cache:<class>:<id>`. The class this included in must
therefore respond to `id`.

add info about runner server requirements

See merge request gitlab-org/gitlab-ce!25242

Also another factor of consideration for
runner server requirements.

Docs: Update all links to relative in /workflow and /administration

See merge request gitlab-org/gitlab-ce!28971

(CE Port) New user onboarding (Skeleton)

See merge request gitlab-org/gitlab-ce!28753