feat: save avatars locally

[LeonTheDev-io]

Versioning information

• This includes major changes (breaking changes)
• This includes minor changes (minimal usage changes, minor new features)
• This includes patches (bug fixes)
• This does not change functionality at all (code refactoring, comments)

Is this related to an issue?

Mentioned in: #336

Changes made

• Added users worker, for handling avatar downloads (src/lib/workers/users.js)
• Added getAvatarData() method for returning the correct server/global avatar hash along with CDN-URL to src/lib/users.js
• Modified src/lib/tickets/archiver.js to use save avatar and use new getAvatarData()

Confirmations

• I have updated related documentation (if necessary)
• My changes use consistent code style
• My changes have been tested and confirmed to work

[eartharoid]

This doesn't need to be a worker (in a separate thread), it can be a normal function.
Avatars are small enough that it doesn't really matter but it would be more efficient to pipe res.body to a writable stream from fs.createWritableStream()