Kibana version: 6.7.0

Elasticsearch version: 6.7.0

Server OS version: n/a

Original install method (e.g. download page, yum, from source, etc.): rpm

Describe the bug:

Despite slack rebranded their logo in the beginning of this year, slack icons on threshold alert configuration page aren't yet followed.

Steps to reproduce:

1. Go to Management > Watcher, Click on Create threshold alert
2. Type in an appropriate name and index pattern
3. Click on Add new action button to show Slack action

Expected behavior:

Show rebranded logos which can be retrieved from here.

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context: