Support reading remote collections

[trinity-1686a]

Current fix of #217 only show follower-only (or equivalent) comments to peoples mentioned in it, which means some peoples allowed to read a comment won't see it, but no people not allowed to can. We should query collections that are in the to/cc field of comments, to also show comments to the followers.
This imply sending signed GET requests as Mastodon offer an "Hide your network" option which hide followers from unauthenticated requests
Querying collection should take place here

Plume/plume-models/src/comments.rs

Line 253 in 80a4dae

vec![]// TODO try to fetch collection

Plume version (and/or commit): 0.2.0