Add support for subtask in create Jira issue modal

[Kshitij-Katiyar]

Summary

• Add support Subtask issue type is not supported while creating an issue from Create Jira Issue Modal

Steps to reproduce:

1. Install your Jira instance
2. Connect your Jira account
3. Run /jira issue create <Issue Name>

For further information, refer to this conversation #993

You might need to remove the below lines as well:

mattermost-plugin-jira/server/issue.go

Lines 538 to 540 in 126ad10

	if issue.Subtask {
	continue
	}