Show specified variables for manual pipelines after pipeline is created

Description

GitLab 10.8.0 introduced the ability for users to specify variables when running a pipeline manually. Once a pipeline is created however, the values of the specified variables are not able to be seen. This can make it difficult to fully reason about what should have happened for a manually created pipeline or where an error took place (e.g. human error through a typo).

If variables are meant to remain secret by default, perhaps instead include a "visible" switch for each row that can be turned on to show a specified variable in the pipeline sidebar.

Proposal

Include a list of variables specified solely for the pipeline in the sidebar. Poorly designed proposal sketch:

image

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/issues/44059

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18440