Add a new field isPinned to the topics table to indicate if a topic is pinned.

[lhitmi]

This issue addresses the pin announcements user story and will be completed in Sprint 1, with high impact and moderate effort. It involves adding the isPinned field to the topics table in the database, ensuring it correctly indicates whether a topic is pinned, which allows important announcements to remain visible at the top of forums. User story is issue #16

[mohamed-elzeni]

This is cancelled since it's already implemented in NodeBB.