[Feature] [QoL] Add the option to cache the thumbnail

[Kazer67]

Instead of loading the thumbnail for the collection from various third-party source, it would be nice to have an option (maybe either per users in the settings or directly servers wide?) to cache them locally when added.

The purpose would give an option, especially for those who host Yamtrack at home, to load directly from the LAN instead of the WAN, if they want (which also would make thumbnail load in the event of a downtime from the third-party website).

For example with the Manga category (but should apply to the rest), thumbnail (chosen randomly for this example) are between 100 to 200KB which can be considered very low especially for those with high bandwidth but this need to be multiplied by the number of Manga in the list which may increase (in my case, with around 42 mangas the network used with external thumbnail is 6,72MB while it only use 736,87KB when I'm blocking the loading of images from third-party).

Maybe using a dedicated folder?

[FuzzyGrim]

In the early stages of the application I did store the images locally, but Django, the framework I use to develop the application, needs something like Nginx to serve the local images when running in production, which made the self-hosting setup a bit more complex.

I will have to think about it, and investigate further if there are other options.