Markdown variable support in comment templates

Release notes

Support for markdown variables in the comment templates.

Problem to solve

As a GitLab user, I would like to be able to use markdown variables in the comment templates.

I use a comment template to approve an MR and assign it to be reviewed by a maintainer. The comment template looks like this

~backend LGTM. 

@maintainer can you please review as a backend maintainer?

/assign_reviewer @maintainer
/unassign_reviewer @halilcoban
/approve

After applying the template, I replace the 2 maintainer parts with the username of the maintainer suggested by danger bot.

It would be better to be able to assign a variable and replace only one occurrence, something like

~backend LGTM. 

[maintainer] can you please review as a backend maintainer?

/assign_reviewer [maintainer]
/unassign_reviewer @halilcoban
/approve

[maintainer]: replacethis

and only replace replacethis with a maintainer, eg @dgruzd. but this causes two issues.

  1. Despite maintainer mention correctly linked (can see if hovered over maintainer), instead of username of the maintainer (dgruzd in this example) it's written the word maintainer
  2. Error: Problem with assign_reviewer command: Failed to find users for '[maintainer]'. Screenshot_2023-07-17_at_11.49.14

Proposal

Markdown variable should be supported, including working with mentions.

Intended users