styles: Add overflow: hidden on .message_content.
styles: Add overflow: hidden on .message_content.

This has two purposes:

1. Prevent stupid stacks of diacritical marks from overflowing into
other messages.  Fixes #7843.

2. Prevent Chrome from collapsing the inside bottom margin with the
.messagebox outside (in a way that Firefox doesn’t).

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulipchat.com>

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