fix: remove incompatible icon_url from Slack postReplyInThread - #5297

Changed pages include:

• index.html - view
• 404.html - view
• about.html - view
• blog.html - view
• casestudies.html - view
• community.html - view
• de.html - view
• docs.html - view
• en.html - view
• finance.html - view

There are more changes not included in the list