Invert the polarity on spoilers +/- button

I am a chump.
Merge pull request #39 from eve-val/feature/spoiler-tags

Invert the polarity on spoilers +/- button
Handle 0-threads case in is_forum_read (fixes #40)

This fix both the problem that empty forums show as unread, and the
error after marking an empty forum as read.
Merge pull request #41 from eve-val/unread-indicator-bugfix

Handle 0-threads case in is_forum_read (fixes #40)