[atlas] New default settings for Atlas measurement form

Hello Atlas users, After an analysis of usage patterns in measurement creation, we have rolled out a couple of changes to the defaults in the measurement form (https://atlas.ripe.net/measurements/form/): - Under timing, the default is now one-off (instead of periodic as before) - Default probe selection is now 50 random, worldwide probes (instead of 10 as before) We hope you find these useful. Please let us know if you encounter any issues. Thanks! Stephen RIPE Atlas UI