Add flags for Australian states and territories

[skissane]

Note that this excludes:

• Australia's external territories, such as Norfolk Island – some of them (including all the inhabited ones) have their own ISO 3166-1 codes, e.g. NF for Norfolk Island
• the internal territory of Jervis Bay Territory (JBT), which lacks an ISO 3166-2 code. (It also has no flag, which makes it kind of irrelevant for this project. It has a minimal population, no great importance, and most people probably don't even know it exists; it is a historical oddity)