Added `noticeboard` app.

[ankur12-1610]

Added a noticeboard app.

[Vikhyath08]

I've made a few minor changes to be looked into. A few things need to be changed further:

• [ ] Create a NoticeListSerializer, which has all the fields of the Notice Model as fields except the description, upvotes and downvotes.
• [ ] In the NoticeDetailView, you must not provide the voters list to the frontend. Instead, make a separate serializer field which simply indicates whether the user has upvoted or downvoted. The frontend should not be able to see which other users have upvoted or downvoted that particular notice.
• [ ] While returning the list of notices in the NoticeListView, the sorting order needs to be changed. The general order is to sort by date, but the notices that are "pinned" need to come first and they also need to be sorted according to date.