Add context for course announcements

[Jeremy Hopkins]

Make course announcements context sepcific so that:

• dashboard shows news from all courses as it does now
• if added to course pages only announcements from that course are shown

testing whether $courseid > 1 is an easy test for this.

[Manoj Solanki]

Fix #19

→ <<cset 775d114dcc16>>

[Manoj Solanki]

Note. With this change, caching doesn't apply when displaying a slider on the course page.