Introduce lazy loading of in-game changelog file(s)

[Garanas]

Description

To finalize it needs: https://github.com/FAForever/fa/issues/6238

But before that you can create your own test data by taking two entries from changelogData.lua and turn them into separate files.

The goal is to use the user-global DiskFindFiles to retrieve all changelog files. The changelog files would live in a directory that does not (yet) exist. You can use the file names to populate the list on the right. When a user selects an item in the list the corresponding changelog is imported via doscript. The changelog is then shown to the user in a similar manner as it is right now.