Various improvements on JournalsTab in the preferences

[calixtus]

Most of it are some simple changes to the JournalsTab to improve consistency in the UI.
Removed also the single new empty pseudoabbreviation, since it produced some bugs and I did not see a benefit (correct me if I'm wrong)

Before:

After:

Someday JournalTabViewModel really should be refactored.

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.