Refactor task configuration.

Extract the map of task type to category from the wgGettingStartedTasks
configuration variable to the wgGettingStartedCategoriesForTaskTypes
configuration variable. This eases supporting category names on
non-enwiki wikis, e.g. see I911759610b88c1478680881f50b031586c75376f.

Make the GettingStarted\ApiGettingStartedGetPages and
GettingStarted\RedisCategorySync classes use the new configuration
variable. Also, make RedisCategorySync::getCategories return
canonicalized category names.

Change-Id: I2251eef7d0dddacc735aa5beb12f1045befa4b39
Merge "Refactor task configuration."