https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9219#note_24032923

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9219#note_23762243
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9219#note_23762268

https://gitlab.com/gitlab-org/gitlab-ce/issues/27762#note_23520780

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9219#note_23436980

in validation. Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9219#note_23437431
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9219#note_23437868

Feedback:

* https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9219#note_23343951
* https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9219#note_23344036
* https://gitlab.com/gitlab-org/gitlab-ce/issues/27762#note_23344797

TODO: Add tests and screenshots

Store the value in application settings.
Expose the value to Workhorse.

This will avoid autoloading issues in the long term.
Signed-off-by: NRémy Coutable <remy@rymai.me>

Also, remove non-existing Ci::ApplicationSetting.expire!
Signed-off-by: NRémy Coutable <remy@rymai.me>

This reverts commit c2093486, reversing
changes made to 4b7ec44b.

This reverts merge request !8573

This reverts merge request !8671

Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

This MR enables rendering of PlantUML diagrams in Asciidoc documents. To add a
PlantUML diagram all we need is to include a plantuml block like:

```
[plantuml, id="myDiagram", width="100px", height="100px"]
--
bob -> alice : ping
alice -> bob : pong
--
```

The plantuml block is substituted by an HTML img element with *src* pointing to
an external PlantUML server.

This MR also add a PlantUML integration section to the Administrator -> Settings
page to configure the PlantUML rendering service and to enable/disable it.

Closes: #17603

If the database has not been migrated properly, the initializer for Sidekiq
will fail to start and cause the whole Rails server to crash. This change
checks the existence for the column and allows Rails to start even
if the setting has not been added as a database column.

Closes #24452

This commit adds a number of _html columns and, with the exception of Note,
starts updating them whenever the content of their partner fields changes.

Note has a collision with the note_html attr_accessor; that will be fixed later

A background worker for clearing these cache columns is also introduced - use
`rake cache:clear` to set it off. You can clear the database or Redis caches
separately by running `rake cache:clear:db` or `rake cache:clear:redis`,
respectively.

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

Refactor and rename `restricted_signup_domains` to `domain_whitelist` to better conform to its behavior and newly introduced behavior.