Fix Flow in response to https://gerrit.wikimedia.org/r/#/c/82527/

* Get rid of 'gettoken' var in API action=flow
* Introduce type 'flow' to fetch edit token from API action=tokens
* Move Flow's edit token salt into global var (so the value is centralised in 1 var)

Change-Id: I104ee54100e151b33890bb29b093d9ee8935eb78