Send monthly digest

[ramikg]

Overview

This PR makes Bolt send a monthly digest on the 1st day of every month:

Bolt automatically finds the channels in which there has been activity in the past month, and sends a personalized digest to those channels.

Some notes

• I updated Go to version 1.22, in order to use slices.Concat.
• At first I thought of adding a new Slack command, but now I think that the monthly variant would have a nicer "social effect". In any case, it should now be simple to implement something similar in a Slack command.
• The code for building the top venues & top hosts parts is pretty similar, but I think that making it generic would make it unnecessarily complicated. This might change in the future in case this feature evolves.
• The number of positions is at most 5 because the maximum number of fields in a section block is 10 (and each row uses two fields).

[ramikg]

Slightly changed digest format following some comments.
(Monthly (Total) instead of Monthly/Total. See here for a visual comparison.)

[ramikg]

I've replaced the "top hosts" category with one that I think is more interesting – the people who've had the most participants in their orders.
(The host is excluded from the count, as well as people who didn't order anything.)

[oriser]

Very nice :)
I think that it must have the ability to disable this behaviour using a configuration env, some people will not want that.
In addition, add the option to customize the channel/s where this message is sent.
I think it will be valuable to allow it also using a slash command.

[oriser]

Please add a test for that

[oriser]

Please add a test for that

[oriser]

Please add a test for that

[ramikg]

Thank you for your comments!

I probably won't be able to get to it in the near future, but I think it'd be nice to have some design decisions at hand in case I get to work on it:

In addition, add the option to customize the channel/s where this message is sent.

Assuming you mean adding a whitelist/blacklist determining which channels receive this message –
Would a whitelist/blacklist be enough, or do you want another configuration option for disabling the feature completely?
(I think the first option would suffice.)

Alternatively, if you mean sending the digest of channel A to channel B, then that might be a nice addition to the proposed slash command.
(I've used this to test this feature, but the code is not in this PR as it doesn't check that the user is part of the requested channel.)

I think it will be valuable to allow it also using a slash command.

Would the output of the command be in the same format as in the screenshot above?
(Although you'd have to replace the month's name with "The past month"/"Past month" for it to make sense.)