Subtasks are not supported in subscription modal

[mickmister]

At the moment, we don't support selecting "subtask types" as "issue types" in the subscription modal. It seems this is on purpose based on this line:

mattermost-plugin-jira/webapp/src/utils/jira_issue_metadata.tsx

Line 63 in a86928d

return project.issuetypes.filter((i) => !i.subtask);

The task here is to look into the feasibility of supporting subtasks in the subscriptions feature. This would also affect the create issue modal if we were to change the line shown above.