feat: improved search engines

[Meierschlumpf]

Describe the feature you'd like to request

Add support for custom search engines by moving them to the manage page and allowing the owner to customize the available search engines.

Describe the solution you'd like to see

Add a new database model for that and allow the admin to add as many search engines as he likes. Those will then be added to the users web category where he can search directly for whatever he needs.

Additional information

See ajnart/homarr#948
and ajnart/homarr#1647