Support quick actions when editing an issue / MR / epic description

Summary

We already support quick actions when editing comments (added in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31136).

We should also support quick actions when editing descriptions.

This is very useful in cases where you need to apply a template after an issue / MR / epic is created.

Some cases where a user needs to edit:

1. Creating an issue from the issue board only allows setting of title.
2. Creating an epic also only has the title field.
3. Creating an MR from git push options

Steps to reproduce

• Create a template that contains /label
• Create a new issue with the template, and notice that /label is stripped
• Create a new issue without a template, say from a board
• Edit the issue, assign the template, and save
• notice that /label is not stripped

Expected behavior

/label should apply the label and be stripped.

Actual behavior

/label appears in the issue.

Relevant logs and/or screenshots