[Suggestions] Much faster imports

[Squareys]

Hey all,

The rate limit hit is actually the 300 requests / minute per webhook, so running the importer per channel in parallel gives a huge speedup. You will need to create a per-channel webhook name for that.

Additionally, messages can be appended to the previous message, if the user matches and no replies, files or reactions are attached to the previous file (remap the thread_ts for these appended messages). Beware of the max message length.

I have a very scrappy diff for this, which I might attach later. It's probably too scrappy for a merge request, though.

Best,
Jonathan