Support autocomplete for commands in group entities

What does this MR do?

It adds autocomplete support for group entities (epics for now):

• it adds commands action to Groups::AutocompleteSourcesController
• it adds commands method that fetches available commands from QuickActions::InterpretService
• it adds some checks to QuickActions::InterpretService to make sure not available commands are not suggested on epics

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #7723 (closed)