Add slack channel notification integration

[GlitchWitch]

Contributions primarily made by @chembrad44 of GlitchSecure

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This PR adds a simple notification integration which will alert a specified slack channel on various actions.

A user must create a Slack App in their Slack instance with the chat:write scope and provide both the SLACK_BOT_TOKEN for the app and the SLACK_CHANNEL_ID for the channel they wish to have notifications arrive in.

Creating a slack app

1. Visit https://api.slack.com/apps
2. Choose "Create New App"->"From scratch"
3. Select your workspace
4. Visit the "OAuth & Permissions" tab
5. Select "Add an OAuth Scope" and enter chat:write
6. Install the app to your workspace and invite the bot to your channel
7. Copy the "Bot User OAuth Token" to the SLACK_BOT_TOKEN environment variable and the Channel ID to the SLACK_CHANNEL_ID environment variable.

[CLAassistant]

All committers have signed the CLA.

[meltyshev]

Hi! Thank you for your contribution 🙏
We'll be sure to take a look as soon as we get back on track after the holidays. Happy New Year 🎄

[jalibu]

Pleeeeeease use that opportunity to add outbound notifications (also email) as a general feature into Planka 🙏

That's just the missing piece for perfection!

[gayathridevii]

Any updates on this?

[meltyshev]

I apologize for taking so long, trying to make time for this. I can't accept right away as I need to make some changes. For example, move notifications sending from controllers to helpers (just to keep the same code structure), use fetch instead of additional axios. We've also thought about being able to send notifications based on the language selected by a user, but it can be added later.

[meltyshev]

Merged: #676

[meltyshev]

Damn, I really apologize, due to squash merge and other PR with refactoring you weren't listed as contributors :( Trying to figure out how this can be fixed...

UPD: fixed 🎉