Refactor querymap-related stuff

* Moved overrides code into own function
* Removed globalized flowTransformMap - it was unwanted in the first place[1]
* Moved map transformation til after overrides are merged (to achieve ^)
* Updated edit conflicts pre-handler; doesn't need own transformation anymore
* Accept multiple overrides, so we can have more generic code (now edit
  conflicts; later also fetching content from editor objects)

1: https://gerrit.wikimedia.org/r/#/c/144471/1/modules/new/flow-api.js

Change-Id: I62d176f9b0fefb61b401cbc0b683b66aaef4d478
Merge "Refactor querymap-related stuff"