Introduce multiple file upload app enhancements

[a1eksb]

This pull request introduces the following Django file upload app enhancements:

• Make file owner property optional: There are use cases where the files are not tightly tied to an owner but to other entities (for example a project in case of BMIP)
• Introduce deleted_on property: In some cases we may want to keep files and simply not show them after deletion. For such cases it makes sense to keep a deleted_on date.
• Introduce max_file_size property: In some cases we want to check the file size and make sure files larger than the max file size are not uploaded.
• Fix a bug where file batch is being accessed via file_upload.batch instead of file_upload.file_upload_batch

Additionally it also introduces a draft of the new documentation.

[a1eksb]

I realized only one reviewer can be assigned so @dameyerdave please also check it out and let me know if you have additional ideas / suggestions.

[a1eksb]

Once merged we also need to add the server entry for docs.

[olloz26]

Commit messages: No space before or after a bracket, e.g. [FEAT] instead of [ FEAT ]. See: https://unlimited.ethz.ch/pages/viewpage.action?spaceKey=nexuswiki&title=Git+Conventions

[a1eksb]

Thanks for the feedback @olloz26 ! I modified my dev environment settings to have all commits with no spaces before / after brackets. 😁

[olloz26]

The [] are not my invention. Do you know https://www.conventionalcommits.org 😉