Maintenance mode testing template

What does this MR do?

This MR adds in a template using which we will open issues for all stages to contribute to crowd-sourced testing prioritization of maintenance mode.

These issues will be opened in addition to a GitLab wide announcement via Slack, which you give you more context:

Slack announcement

Crowdsourcing maintenance mode testing

:waves: Hi everyone,

The Geo team has finished the implementation of the MVC for a maintenance mode. 

While GitLab is in maintenance, no write requests can be made by non-admin users. Users can login and perform other read operations. You can read more about the feature here. 

Maintenance mode is a top requested feature by sys admins and an important step to improve our ability to provide a simple disaster recovery solution. 

Before we ship this iteration in 13.9, we need your help! Maintenance mode affects the entire application and we want to ensure that we’ve not overlooked any features that are still able to write any data. We’ve already conducted exhaustive tests but found nothing so far!

Goal
Get a list of the most important behaviours of the features you develop, that we should test. We want to ensure that the first iteration does not break any existing features.

Who and how? Everyone can contribute!

Engineers/managers know their parts of the product most intimately and can quickly and more thoroughly estimate what their features should behave like in maintenance mode.

We will be opening issues for all stages and assigning them to it’s Product or Engineering manager. Please collaborate with your team and complete the short list of requests in it.

When? To be able to ship in 13.9, we ask you to complete manual testing by 2021-02-05

We will not delay the release unless a blocking bug is found in this test exercise.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team