feat: Unapproved mods download

[Alystrasz]

This PR enables downloading mods that have not been previously approved (verified).
A new UX path is introduced for mods presenting a DownloadLink member in their mod.json file.

1. Such mods are displayed as potentially downloadable in the server browser (yellow sign, as opposed to a green one for approved mods):

2. When joining a server requiring unapproved mods, user is prompted for permission:

Keep in mind this is hypothetical for now, as masterserver does not send mod download information to clients yet.

TODOs

• Wait for Atlas to send mod download links through client/servers resource
• Test this PR with real usecases

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Part of #883:

Servers send a list of Mod-URL pairs to clients, which clients then download the mods from.