Add possibility to assign groups to newly added entries from the beginning

[DL6ER]

What does this implement/fix?

See the related Discourse Feature request for screenshots: https://discourse.pi-hole.net/t/add-group-management-dropdown-to-add-a-new-domain-or-regex-filter/74696

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Related issue or feature (if applicable): https://discourse.pi-hole.net/t/add-group-management-dropdown-to-add-a-new-domain-or-regex-filter/74696

Pull request in docs with documentation (if applicable): N/A

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2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
• I signed off all commits. Pi-hole enforces the DCO for all contributions
• I signed all my commits. Pi-hole requires signatures to verify authorship
• I have read the above and my PR is ready for review.

[rdwebdesign]

Depending on the browser window size, the new field can overflow, hiding part of the control:

To avoid this issue, I added a new commit to use different classes for large and medium screens (col-lg-3 col-md-6):

[yubiuser]

Not sure if this is on purpose, but the group selector does not reset to default after a client/domain/list is added with a manually changed group selection.

[DL6ER]

Yes, this is on purpose. During playing around I found it quite counterintuitive that the field resets, e.g., when I add multiple domains but don't add them at once (space separated) because I want different comments for each of them.