fix #988 sort numerically

[kagahd]

As written in #988 blog notes of older photos (before 1970-04-27) are wrongly sorted shown.
The reason becomes obvious when looking at the numerical presentation of the dates:

• 1970-04-26 = 9936000000
• 1970-04-27 = 10022400000
• 1970-11-01 = 26265600000

I first suspected the culprit in the “10-digit rule” which is to distinguish seconds from milliseconds when normalizing timestamps. In many systems, a timestamp value less than 10^10 is assumed to be in seconds, whereas a larger value is assumed to be in milliseconds. But this assumption did not hold true since PiGallery2 uses dates properly transformed to milliseconds.
The reason is even simpler, but well hidden in the blog.service.ts.
The line dates.sort(); does not look suspicious at all unless you know that the JavaScript sort() method sorts values lexicographically (as strings) rather than numerically even if dates is an array of type number. This leads to incorrect sorting of timestamps. The solution is to provide a numeric comparison function like sort((a, b) => a - b) to ensure proper chronological sorting, which this PR has implemented.

[bpatrik]

ah nice finding. Thanks!