R1:a226d7413383

Use "rest/api/3/myself" to retrieve JIRA profile details, not "rest/auth/1/session"

Summary:
Ref T13493. At time of writing, the old API method no longer functions: `1/session` does not return an `accountId` but all calls now require one.

Use the modern `3/myself` API instead. The datastructure returned by `2/user` (older appraoch) and `3/myself` (newer approach) is more or less the same, as far as I can tell.

Test Plan: Linked an account against modern-at-time-of-writing Atlassian-hosted JIRA.