#282 - Slack notification with approval buttons

[EwelinaSkrzypacz]

This should close #282.

For now I made it in a simple way - use things that we already have in our codebase. In the future we should make it using Slack Block Kit and Laravel package laravel/slack-notification-channel - I've made an issue for that #348.

For technical approver it looks like this:

For administrative approver it look like this:

When user clicks for example button "Rejected", but the request is already rejected, he will see information about this:

In Slack API on production we have to update two things:

1. In Slash Command tab the URL is changed from www.your-url-here.com/api/slack to www.your-url-here.com/api/slack/commands
2. In tab Interactivity & Shortcuts the option Interactivity must be enabled and request URL should be www.your-url-here.com/api/slack/actions.