[Forum] The "Image" and "Mixed" forum types should be able to accept multiple images.

[jannes-io]

What problem would you like to solve?
It's a little bit of a hassle when you want to show off multiple images to create multiple topics, or 1 topic and then a comment with additional images.
This change should allow users to upload multiple images to their topics.

Additional context

1. The file upload we have now does not support multiple uploads, this would also have to be made (it currently extends from Symfony's filetype, which does have a multiple option, we just ignore it atm..),
2. The forum/topic entities have already been modified to allow multiple images, it's just not possible to input/display right now,
3. The images should be displayed in the image album stim controller we already have (I know it's basic atm but we can improve it later),
4. It should also be possible to remove, or add more images when editing the topic.