Submission threads for pending mods

[Fleeym]

This PR adds support for forum-style threads for pending mod_versions.

Current flow:

• When a mod version that will become pending is created, a submission will be automatically created
• Only mod developers and index admins can comment on the submission
• A comment can have at most 5 attachments (photos). Accepted formats currently are .png, .jpg, and .webp. Regardless of uploaded filetype, the attachment will be converted to .webp to preserve space server-side.
• Submission threads can be locked by admins
• Comments and attachments can be deleted by admins and comment authors

Technical stuff:

• Files are stored on the filesystem, instead of in the database (like mod logos are currently)
• Static file serving will be handled by nginx, index doesn't serve them
• This PR technically adds a file storage directory for the index, we can add more stuff here (move mod logos, store about.md attachments)

Todos / stuff to consider:

• Lock submission threads when the mod gets accepted
• Make comments pinnable, useful to pin the final comment in a thread, to easily see the conclusion of the submission
• Maybe add support for more image formats on attachments
• Maybe look into having markdown support for comments (requires careful sanitization, ammonia should have all we need but it needs to be configured)
• See if using tokio::task::spawn_blocking is fine for image conversion
• Image moderation - if the submissions are public, even if readonly, we'll have to be careful about what images people post, it's the internet after all
• Notifications for submission comments (probably setup a webhook here, should be fine)
• Consider an ACK system for submissions: a certain number for ACKs will automatically approve the mod; could also be a full-on replacement for the current system