completion for @mention in comments

[stfl]

We're using jira comments to communicate within the team which requires mentioning the user with @username
In org-jira this is turns into [~accountid:<id>]
I'd love to have a completion function to select the username and insert the accountid

this should work with (org-jira-get-assignable-users) but my elisp experience is quite limited.
Maybe someone already has a function for that and is willing to share.

[ahungry]

If anyone does, feel free to make a PR, this would be a nice addition.