message_list: Use document.getSelection to identify drags.
message_list: Use document.getSelection to identify drags.

This removes the messy click-or-drag detection algorithm originally
added in fe8f63c3896ae24da6b8f83feb89b1bf6711449b, which fixed a messy
bug in an earlier algorithm from ~2013, whose sole purpose as to check
whether we're doing a selection and if so, not trigger the
click-on-message-body click handler.

The right fix is of course to do that check correctly.

brainwane/zulip,kou/zulip,brainwane/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,rht/zulip,kou/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,zulip/zulip,synicalsyntax/zulip,shubhamdhama/zulip,synicalsyntax/zulip,shubhamdhama/zulip,zulip/zulip,eeshangarg/zulip,hackerkid/zulip,zulip/zulip,timabbott/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,punchagan/zulip,shubhamdhama/zulip,kou/zulip,timabbott/zulip,shubhamdhama/zulip,rht/zulip,hackerkid/zulip,timabbott/zulip,timabbott/zulip,shubhamdhama/zulip,hackerkid/zulip,synicalsyntax/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,punchagan/zulip,synicalsyntax/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,punchagan/zulip,andersk/zulip,showell/zulip,synicalsyntax/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,brainwane/zulip,zulip/zulip,kou/zulip,shubhamdhama/zulip,showell/zulip,timabbott/zulip,brainwane/zulip,showell/zulip,synicalsyntax/zulip,punchagan/zulip,brainwane/zulip,rht/zulip,synicalsyntax/zulip,andersk/zulip,shubhamdhama/zulip,kou/zulip,showell/zulip,hackerkid/zulip,showell/zulip,timabbott/zulip,showell/zulip,timabbott/zulip,brainwane/zulip,andersk/zulip,kou/zulip,brainwane/zulip