Clean up incorrect use of regular expressions in CategoryInputWidget

getLookupMenuItemsFromData() constructed a regex from user input
without escaping. I don't *think* there are any injection
vulnerabilities here but at the very least it triggers exceptions
when the input is, say, a backslash. Instead, use .lastIndexOf() which
allows us to efficiently check whether a string starts with a certain
prefix.

getLookupCacheItemFromData() was stripping out the Category: prefix
using a regex that hardcoded Category: (so failed to detect localized
prefixes) and used global replacement, which meant that strings with
multiple occurrences of 'Category:' were handled incorrectly. Instead,
use mw.Title to strip the prefix. Also move away from .map() because we
may need to drop a result if it doesn't pass mw.Title validation.

this.categoryPrefix still has a few legitimate uses left, so keep it
around but set it to the localized namespace prefix rather than Category:

Change-Id: I6547f9df2e94fe81f6aefb9286e547425137344b
Merge "Clean up incorrect use of regular expressions in CategoryInputWidget"