Allowed for zero-weight, negative-weight and multiple split tests conversions

- Added migration script to start the conversion weights at 0
- Now adding up successive conversion weights instead of overwriting them
- Allowed for zero-weight and negative-weight conversions, adjusted significance calculation accordingly
- Adjusted tests
Formatting
Only allowed for multiple conversions if a weight is explicitly given

- Added tests
Merge pull request #1467 from fauvel/issue-1467

Allow conversion weight 0 in split tests