Add commit message body as variable in custom changelog template
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### Release notes
New variable "commit.message_body" for custom changelog templates
### Problem to solve
The changelog generation function allows [custom templates](https://docs.gitlab.com/ee/user/project/changelogs.html#custom-templates). The currently available commit data fields are commit.title (as title), commit.reference, and commit.trailers. The main body of the commit message is not available.
### Proposal
Please also support the **commit message body** (the lines between 'title' and 'commit.trailers' with leading and trailing blank lines stripped). Proposed variable name: commit.message_body
### Intended users
Release Manager, Application Ops
issue