Support displaying spoilers
This adds support for displaying recieved spoilers, but not sending them. Spoilers are displayed as a black rectangle, and can be clicked on to be revealed. If the last message in a channel was a spoiler, it is not shown on the left sidebar.
The spoiler blackening is done in CSS, but to check if a message
contains a spoiler for determining if it should cause a different cursor
to be displayed and if it should be shown in the sidebar, a simple check
of if the message contains data-mx-spoiler
is used.
Contributes to #335 (closed).
Edited by Smitty van Bodegom