FE: Work item description templates from many sources

In !174572 (merged), some placeholder Work Item description templates were added in the frontend while the backend for the GraphQL API was implemented.

When the backend is complete, we should remove the placeholder templates from app/assets/javascripts/graphql_shared/issuable_client.js so that the GraphQL Client fetches them from the backend.

The backend is being updated to include a category with each result in the list. We should also group together templates in the same category and present them as grouped items in the dropdown.

Requirements

• When viewing a list of work item description templates, the real templates for the project/group are displayed, rather than some placeholders.
• Remove the requirement for the work_items_alpha feature flag to be enabled for the templates section to be visible
• Update the documentation to include information on how to use the description templates feature.
• Group together templates by category in the list

Implementation

It should simply be a case of removing the code added here: issuable_client.js