Add option to change number of threads for imagemagick convert

[caplod]

This PR …

If you are converting a lot of big images with imagemagick it can get really slow.

This PR adds an option to configure the threads imagemagick can use. (default is 1)
So you can benefit from capable hardware.

Ready?

• Unit tests for fixed bug/feature
• In-code documentation (wherever needed)
• Tests and checks all pass

For review team

• Add changes to release notes draft in Notion
• Add to website docs release checklist (if needed)

[bastianallgeier]

I know that the option in imagemagick is called thread. But I feel like the option should still be called threads. @distantnative @lukasbestle could this be something that would actually also help for @Iskris' issue?

[caplod]

Yeah, I thought that too. I changed the name of the option into 'threads'

[bastianallgeier]

I'm happy with it. But it would be good to get a second opinion (@distantnative @afbora @lukasbestle)

[lukasbestle]

Makes a lot of sense to me. I think the original limit was added for shared hosting setups, but if the hardware is available, using multithreading can indeed improve the performance a lot.

[bastianallgeier]

Thanks for your help @caplod We will make sure to mention you in the release notes.