Cross-proposal asset links do not work

[SamWilsn]

If you create a link like [some asset](../01234/assets/foo.pdf), Zola will output a link to the wrong URL because we make heavy use of slugs/aliases/paths.

Related to:

• Make @/ links resolve for assets too getzola/zola#3094
• Add internal resolution for assets in get_url function getzola/zola#3065
• Add internal resolution for assets in get_url function getzola/zola#3065