Post a message to Slack after release flank

[piotradamczyk5]

Author the user story for this feature

As a Developer, I want to post a message to Slack after each release so I do not need to do it manually 😄 .

Is your feature request related to a problem? Please describe.
Now after each release developer must post a message to firebase slack channel with info about new release.

Describe the solution you'd like
Add step at the end of release job which will post a message

Additional context
The message should look like:

Flank <version> has been released:
<release notes>

[bootstraponline]

I love this idea!

[Sloox]

Easy to do but need to ask for permission from the Firebase slack channel @bootstraponline do you perhaps know who to contact? I need to either install a webhook or a bot depending on which way we want to install the auto flank release notifier.

[bootstraponline]

Maybe Paul? He'd redirect us to the best person to talk with.

The other thought is creating a Flank slack user, adding the user to Slack, and then using a token associated with that user account.

[Sloox]

@bootstraponline It appears i can request installation. I will see what happens

[Sloox]

An inital release for testing purposes is done : #1058
Will wait for permissions then close this once i can

[Sloox]

Pushed into ranked as we are still waiting. There is a test push to our slack channel once a release occurs. Will be waiting for that.
Slack app link: https://api.slack.com/apps/A019J856KKL/install-on-team?
Send To Slack integration: https://github.com/marketplace/actions/send-message-to-slack

[Sloox]

Needs some work as the formatting is incorrect and looks ugly
EG:
Flank "v20.09.0" is released \n ## v20.09.0 ### Bug Fixes - #1087 Fix release action () - #1080 Flaky Flank-scripts test (Sloox) - #1079 Fix slack notification in slack (Sloox) - #1054 Firebase refresh fails when test zip file doesn't exist (#1052) (axelzuziak-gogo, jan-gogo) - #1070 Avoid fast fail while creating a bucket (adamfilipow92) - #1066 Bad validation of local-result-dir by Doctor (piotradamczyk5) ### CI Changes - #1058 Post slack message about releases (Sloox)

[bootstraponline]

@Sloox I don't think Firebase will ever approve integrations because they are on the Free plan and there's a limit. To successfully deploy this, I expect we'll need to create a Flank bot user account on Firebase and use the token from this user account.

[Sloox]

@bootstraponline I have already done as you mentioned. I am waiting for a response as when you add a bot onto firebase you must request its permission to be added. Then for us it is as simple as changing one key.

[Sloox]

Alright after further talks with @bootstraponline and the fact that the bot will most likely not be granted permission.
Will look into getting this to work by using an xoxc token and a flank "bot" user.
The xoxc token does not work with the github actions and will need to be changed accordingly.
If it requires some scripting it will add it to flank_scripts.
However will first see if there is an easier solution such as where we can simply provide the token and send a message.