Refactor the Heading button section, in order to put a chat button in there

[HMarzban]

There is a problem here, @edsaperia I need your help:

Each heading contains two buttons right now:
1/ # for copying the id of the heading into the clipboard.
2/ A toggle button, which is for folding and unfolding the section

Where should I add a new button for the chat? How should these three buttons be arranged?

[edsaperia]

1. Heading id should go into the address bar when you click on the heading in the table of contents. No need for a button to copy onto clipboard.

2. Toggle button stays as it is, but should only be visible on hover (see Toggle icons visible on hover only #39)

3. Chat icon should be to the right of the heading in the margin of the document. If there are no unread chats, it should be a chat bubble, if there are unread chats it should be a number in a circle.

Unread chats should also be shown in the table of contents in blue circles, similar to slack:

[edsaperia]

note that the designs of the icons are approximate - the images above just indicate the locations of the buttons.