download avatars during install time

[Marketen]

• When a package is installed, the avatar PNG is downloaded alongside the Docker image and saved to [REPO_DIR/avatars/{dnpName}.png] or [DNCORE_DIR/avatars/{dnpName}.png]. Depending if its core or not
• The Express server serves these directories at /avatars/, like my.dappnode/avatars/rotki.dnp.dappnode.eth.png to get the rotki avatar
• When listing installed packages, [resolveAvatarUrl()] detects the new local file inside the repo and returns /avatars/{dnpName}.png instead of a remote IPFS gateway URL
• Already-installed packages (before this change) continue using IPFS gateway URLs until re-installed — fully backward compatible

packages installed before this PR still go to avatar URL, new package rotki is served through http://my.dappnode/avatars/rotki.dnp.dappnode.eth.png

[github-actions]

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