Add option for course announcements days

[Jeremy Hopkins]

Add a separate option to allow timeframe (days) of course announcements to be set independently of the days we wish to display site announcements for.

[Manoj Solanki]

Partly fixed by this commit. https://bitbucket.org/covuni/moodle-block_news_slider/commits/15f829e8ca143ee03baf4f8ec334169b73730b1d

[Manoj Solanki]

Fix #23 and Fix #24. Final commit following testing. This change fixes an issue with posts not displaying within the time period that has been set.

→ <<cset f0f71fb0507d>>