Koding: #index: landing page for Koding integration

If enabled it will provide a link to open remote Koding instance url
for now we are also providing the sneak preview video for how
integration works in detail.

Repository: check whether .koding.yml file exists on repository

Projects: landing page: show Run in IDE (Koding) button if repo has stack file

Projects: MR: show Run in IDE Koding button if repo has stack file on active branch

ProjectHelpers: add_koding_stack: stack generator for provided project

With this helper we will auto-generate the required stack template
for a given project. For the feature we can request this base template
from the running Koding instance on integration.

Currently this will provide users to create a t2.nano instance on aws
and it'll automatically configures the instance for basic requirements.

Projects: empty state and landing page provide shortcuts to create stack

projects_helper: use branch on checkout and provide an entry point

This ${var.koding_queryString_branch} will be replaced with the branch
provided in query string which will allow us to use same stack template
for different branches of the same repository.

ref: https://github.com/koding/koding/pull/8597/commits/b8c0e43c4c24bf132670aa8a3cfb0d634acfd09b

projects_helper: provide sha info in query string to use existing vms

With this change we'll be able to query existing vms on Koding side
based on the commit id that they've created.

ref: https://github.com/koding/koding/pull/8597/commits/1d630fadf31963fa6ccd3bed92e526761a30a343

Integration: Docs: Koding documentation added

Disable /koding route if integration is disabled

Use application settings to enable Koding

Projects_helper: better indentation with strip_heredoc usage

Projects_helper: return koding_url as is if there is no project provided

current_settings: set koding_enabled: false by default

Koding_Controller: to render not_found once integration is disabled

Dashboard_specs: update spec for Koding enabled case

Projects_Helper: make repo dynamic

ref: https://github.com/koding/koding/pull/8597/commits/4d615242f45aaea4c4986be84ecc612b0bb1514c

Updated documentation to have right format

Fix escaped angle bracket's in integration documentation.

There are several escaped angle brackets in our integration docs.  While these
render fine within GitLab, they are broken when rendered on doc.gitlab.com because
pandoc does not escape them correctly.

You can see the problem here: http://doc.gitlab.com/ce/integration/github.html

Note that the strings `<Organization>` and `<Your Name>` are being interpreted as html tags.

It looks like doc.gitlab.com is using pandoc: https://gitlab.com/gitlab-com/doc-gitlab-com/blob/master/generate.rb#L64

You can verify these changes by running something like this and then inspecting `test.html`:

    pandoc --from markdown_github-hard_line_breaks -o test.html doc/integration/twitter.md

You can also verify that GitLab continues to render the docs correctly by checking, for example, http://localhost:3000/help/integration/twitter.md


See merge request !4128