REST API issueLink should not accept jira_subtask_link type

Steps to reproduce

1. create a json file with this content:

   {
       "type": {
           "name": "jira_subtask_link"
       },
       "inwardIssue": {
           "key": "DEMO-1"
       },
       "outwardIssue": {
           "key": "DEMO-2"
       },
       "comment": {
           "body": "Linked related issue!",
           "visibility": {
               "type": "group",
               "value": "jira-administrators"
           }
       }
   }
   

2. call the REST end point /rest/api/2/issueLink

Unexpected Behavior

• DEMO-2 becomes a sub-task of DEMO-1 unconditionally:
  • there's a breadcrumb in DEMO-2's view screen showing DEMO-1 as its parent
  • in DEMO-1's view screen, DEMO-2 is under the Sub-tasks section
  • in the db, there's an entry written into issuelink table reflecting this link (except that "sequence" column is NULL)
• However, the original issue type of DEMO-2 is unchanged, so it may still be a standard issue type like Bug, or even an Epic
  in case of Epic, DEMO-1's Sub-tasks section throws an error: epic.txt

Suggestion

Since issue type can't be updated using this end point, it should not accept jira_subtask_link type.

Something similar might also happen to Epic-Story Link.