Closes #25950

[ci skip]

The button allows to choose a ".gitlab-ci.yml" template that automatically
sets up the deployment of an application.
The currently supported template is Kubernetes template.

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

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

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

Sometimes admins will change the LDAP configuration, not realizing
that problems will occur if the user's LDAP identities are not
also updated to use the new provider name. This task will give
admins a single command to run to update identities and will
prevent having to run multiple Rails console queries.

This fixes a compatibility issue with Git 2.11:

- Issue: gitlab-org/gitlab-ce#25301
- gitlab-shell MR: gitlab-org/gitlab-shell!112
- CE MR: gitlab-org/gitlab-ce!7967
- EE MR: gitlab-org/gitlab-ee!964
Signed-off-by: NRémy Coutable <remy@rymai.me>

This commit introduces the concept of deployment variables - variables
that are collected from deployment services and passed to CI runner
during a deployment build.
Deployment services specify the variables by overriding
"predefined_variables" method.

This commit also configures variables for KubernetesService

In a high availability configuration, the Redis and PostgreSQL nodes
should not attempt to run database migrations. In fact, trying will
result in errors about not providing a database password. To
prevent errors and confusion, add this configurtion to these nodes'
`gitlab.rb` file.

[ci skip]

[ci skip]

Update Mattermost slash commands docs to explain how to create a newline and use <kbd> for user input.
See HTML5 spec: https://www.w3.org/TR/html5/text-level-semantics.html#the-kbd-element