Rename GitLab 'Slash commands' to 'Quick actions'

Resources

Background

We have three main areas of functionality that involve typing text and getting various systems to react. Let's have a consistent naming scheme as we iterate on functionality.

Naming changes

What	Current name	Change (or no change)
Slash commands inside GitLab description and comment boxes	GitLab slash commands	Rename to `Quick actions`, and also absorbing `@`-mentions and future UI paradigms (https://gitlab.com/gitlab-org/gitlab-ce/issues/28962 + https://gitlab.com/gitlab-org/gitlab-ce/issues/28737) into this category
Slash commands inside Mattermost	Mattermost slash commands aka chat commands	Maintain `Mattermost slash commands` name and deprecate `chat commands` name
Slash commands inside Slack	Slack slash commands aka chat commands	Maintain `Slack slash commands` name and deprecate `chat commands` name

Changes required in product and docs

Time tracking area should be updated use quick actions.
Comment box should refer to quick actions.
Docs should be updated, including explaining how to use quick actions and time tracking.

Original description

Having three variants of Slash commands with https://docs.gitlab.com/ee/user/project/slash_commands.html and https://docs.gitlab.com/ee/project_services/mattermost_slash_commands.html and https://docs.gitlab.com/ce/project_services/slack_slash_commands.html that links to https://docs.gitlab.com/ce/integration/chat_commands.html is confusing.

I think we should try to consolidate or solve this situation. Some ideas:

Rename things you do in an external chat client Chat commands
Consolidate all Chat commands documentation and configuration: it should look, feel and work in the same way (this means documentation, in app configuration and usage)
Rename Slash commands in GitLab to Quick actions (I'm not sure about this one, slash commands describes them pretty well, but it's an idea)

@markpundsack @victorwu this falls under your shared responsibility. What do you think?

/cc @DouweM @smcgivern