Support Aircraft Revisions to avoid overwriting later changes

[ericberman]

To test:

• New app vs. old server - create and edit aircraft
• New app vs. new server - create and edit aircraft, verify that you can't edit wrong version and can edit correct version
• old app against old server - no point; status quo, so assumed to work
• old app against new server - create and edit aircraft, verify that you can add aircraft and can edit for now, but later get "please update" message.