Better indicate comment depth

[eevee]

In a thread like this:

comment
    comment
        comment
            comment
    comment

...it's kinda hard to see what that bottom comment is actually replying to. Often it looks like it starts an entire new thread. Do something about this.

Some options that are not mutually exclusive:

• Light lines indicating the tree structure
• Allow collapsing a thread once read
• Draw a rule between separate threads (eh)