Rename conversion graph components to journey graph

For consistency between module name and corresponding visualisation.
Remove hard-coded property names from Journey graph.

Fetch data from Journey collections based on the graph view's `valueAttr` property instead of using hardcoded value of "uniqueEvents".
Remove hard-coded "uniqueEventsNormalised" property from data points, in favour of calculating the percentage values dynamically.
Add a `max` method on journey-series collections to calculate percentages of maximum values.