Upgrade bundled flot to 0.8.3

Upgrade flot to 0.8.3:

  - update client code i.e. for date formatting to match flot version
  - use flot canvas to allow snapshot export from client with full formatting (server export still in place for those who need it)
  - added ticklabel wrapping to flot canvas plugin (should be obsolete with flot 0.9, see https://github.com/flot/flot/issues/1270)
  - speedup drawing by using flot legend placement
  - legend is now fully customizable via options.js