Add slack notifier

[p0psicles]

• PR is based on the DEVELOP branch
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TODO:

• Modify the sprite and add the css for the slack icon.

#2345

[fernandog]

@neoatomic what tool do you use to edit/add sprites?

[neoatomic]

@fernandog Nothing special really. Just Paint.net and the copy/past, resize function. ;)
And off course check that the size/start pixel is the same as in the css code for the sprite.