message-view: Increase spacing for lists coming after paragraphs.
message-view: Increase spacing for lists coming after paragraphs.

Lists that were followed by a paragraph (i.e. our p+ul, p+ol CSS rule)
in messages had negative top margin of -3px.  Adjusting the margin
here is important, because the default styling would result in an
excessive gap that made bulleted lists weirdly far from the previous
paragraph.  See #12113 for background.

However, the -3px negative margin was so large that it reduced spaces
between paragraph and lists, such that there was too little visible
separation between the two.  We fix this by going with a 0px
margin-top instead.

This has been tested for various structures of messages:
1. text + bulleted list
2. bulleted list + unbulleted list(or two lists)
3. only list.

And it looks good in all cases.

Fixes #17284.

hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,rht/zulip,rht/zulip,kou/zulip,zulip/zulip,andersk/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,kou/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,zulip/zulip,kou/zulip,punchagan/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,punchagan/zulip,rht/zulip,punchagan/zulip