telegram occasionally changes the upstream urls at which images are
served, with old/new partially overlapping in time afaict.
with this commit, a new endpoint is added to tgsa at
/img/$user/$post_id/$img_idx
which serves a redirect to the actual image url that telegram had
returned within the last hour (i.e. as per the cache).
next step is to use these image urls in the bbcode itself.
Change-Id: I9aa5cb56bc444cbe796868346c67f2e1e1b79413
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5610
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
b308361a73
