Canonicalize excluded category names

The category names in the wgGettingStartedExcludedCategories
configuration variable need to be canonicalized like those in the
wgGettingStartedCategoriesForTaskTypes configuration variable (see
I2251eef7d0dddacc735aa5beb12f1045befa4b39).

Use Title::makeTitleSafe and then Title#getDBkey to canonicalize
category names in both PageFilter#getExcludedCategories and
RedisCategorySync::getCategories.

Change-Id: If3c0559cdb4a0bdb372d1fefc8477894005880e3
Merge "Canonicalize excluded category names"