* Join slack channels with slack-id

Fixes https://github.com/metabase/metabase/issues/23229

We upload images to a channel and then send messages to the desired
channel referencing those images from the channel we uploaded. But the
slack bot must be in the channel to upload images.

We handle this in `slack/upload-image!` where we watch the error message
and join the channel if we recognize that is our issue. This special
upload channel is set in the admin section when setting up, typed in by
a human.

Slack now requires us to use the internal id of the channel when
joining. IE we used to use "metabase_files" but now we need to use
something like "C87LQNL0Y23". But we do not have this information and we
don't want humans to have to look this up.

SOLUTION:
change our cache. We currently just get a list of channels and users
```
["#general" "@dan" ...]
```

Change this to
```
{:version 2,
 :channels [{:display-name "#random",
             :name "random",
             :id "CT2FNGZSRPL",
             :type "channel"}
            {:display-name "#general",
             :name "general",
             :id "C87LQNL0Y23",
             :type "channel"}
            {:display-name "@dan",
             :type "user",
             :name "dan",
             :id "UR65C4ZJVIW"}
            ...]}
```

Now we have slack internal ids present. When we attempt to join the
slack channel, look for this id and attempt to use that.

This has some knock-on effects. The UI still lists the channels in a
channel picker when sending pulses. The list sent over the wire still
mimics the previous shape (a flat list) and the choice is still the
human readable name.

In the future we should switch over to using the stable ids rather than
solely channel names. Channel names can be renamed.

I didn't go down this route because of the files channel. It is set at
setup before we have a channel list. We could do some kind of run time
migration but it is difficult because it would change the type of
`slack-files-channel` from string to :json to handle the more complex
type. Or perhaps we could make another setting to hold the json form and
set that when we can positively identify things.

In either case, these changes were not required yet to fix our slack
issue. We just upgrade the information we have about slack channels,
downgrade it when it hits the wire so the UI needs no changes, and use
the extra information in the one spot where we need it.

The cache is populated at startup and every four hours after that. So we
do not need to worry about the old cache shape. If the new code is
running, its the new cache.

* Send #channel and @user forms over wire

We store `{"channel": "#slack-pulses"}` in the pulse_channel.details
column so we should keep those types of values around.

We use the bare portion ("slack-pulses") rather than with the hash on it
so we seem to be mixing usernames and channels. But these sets are
distinct and you cannot create a channel with the same name as a
user. Also, channel names are lowercase while channel-ids are uppercase
so those are also non-overlapping sets.

* Put slack token so slack reports as configured

* Errant tap>

* Alignment and docstring fixes

* Remove slack-cache version information

remove the `:version 2` from the cache. We are always in charge of the
cache, and we compute it on startup so there's little risk of other data
shapes being present.