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.

Maniphest Tasks: T13493

Differential Revision: https://secure.phabricator.com/D21170