Make infoMessages translatable

Summary: Currently, infoMessages from .kbstyle files are untranslatable. This patch allows extracting and translating these messages.

Test Plan:

1. Compile and install KBibTeX.
2. Use extract-messages.sh script to extract messages and translate the new message.
3. Move the new translation to its place.
4. Open some BibTeX file with URL or DOI info and see the message translated.

Screenshot for uk (Ukrainian) locale:

Reviewers: KBibTeX, thomasfischer

Reviewed By: KBibTeX, thomasfischer

Subscribers: thomasfischer

Differential Revision: https://phabricator.kde.org/D24632