[REQUEST] Reminder notifications should be sent via Slack, not via email

[etagwerker]

Before we start...:

• I checked the documentation and didn't find this feature
• I checked to make sure that this feature has not already been requested

Branch/Commit:

main branch.

Describe the feature:

As a Noko user
I forget to submit my time entries
And then I get a Slack notification
And I get the opportunity to make it right

Problem:

Email notifications can be easily ignored. Slack notifications not so much.

Resources:

Smart notifications (#98) introduced an integration with Slack. I believe we should go ahead and use that.

Email notifications are over here:

https://github.com/fastruby/pecas/blob/main/app/models/user.rb#L8-L16

And here:

https://github.com/fastruby/pecas/blob/main/app/mailers/reminder.rb#L4-L9

I will abide by the code of conduct