By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes, and have included unit tests where possible.
• I am willing to help maintain this change if there are issues with it later.
• I give this submission freely and claim no ownership.
• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)

What does this PR aim to accomplish?:

Allow users to add multiple groups/clients/domains/adlists at once by space-separating them. All added items will receive the same comment (if specified).

Example 1

resulting in:

Example 2

resulting in:

Also, we remove the redundant list management from the settings page as this code was only partially migrated to the database backend (it was planed to be replace, anyway) and still uses CLI callbacks in the background. It is not needed any longer. I added a link to the new page so users will not be confused by a sudden absence of the settings tab.

How does this PR accomplish the above?:

Implement processing of space-separated items as individual entries.

What documentation changes (if any) are needed to support this PR?:

None