Change string keys

[jessegreenberg]

For #30 and dependent on #26. I would recommend making these keys a little more friendly for translators.

  "force-description-pattern-target_source": {
    "value": "Force on {0} by {1}"
  },
  "force-description-pattern-target_source_value": {
    "value": "Force on {0} by {1} = {2} N"
  },

source and source-value don't really signify the purpose to me. @mbarlow12 what do you think about
forceOnObjectByObjectPatternString and
forceOnObjectByObjectWithUnitsPatternString

Do you know if these keys were copied from gravity-force-lab strings or did we come up with them as we made ISLC?

[mbarlow12]

Works for me. Would forceOnObjectByOtherObject be too verbose/redundant?

[mbarlow12]

Completed in 303b54c, but I suppose we can wait to tackle #26 until we're closer to releasing the sim. I'll keep my assignment.