Show publication time of RSS news in user’s preferred short datetime format

The publication time of the news items on Krita’s welcome page is now shown in the user’s timezone and in their preferred short datetime format. The timezone of the datetime in the RSS feed is now also taken into account when parsing the publication date (the old code basically assumed that the user lived in the timezone the RSS feed generator was set up to use, which at the time of writing was UTC).

Note that old code only displayed the date; the new code also displays the time (typically hh:mm).

Test Plan

Start Krita. Take a look at the publication times of news items on the welcome screen. (Compare them with the ‘pubDate’ values at https://krita.org/en/feed/, taking time zones into account.)

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.