[Feature Request] Sticky/Pinned Threads
The ability to mark threads as sticky/pinned and view them is highly useful for both moderators and users.

In order for users to get value out of a forum, there needs to be some way of consistently accessing reference material so that new users can learn about common issues/features/tutorials and also so that more experienced users can generate helpful content for new users easily, without needing to create a blog (or website), or file a pull request to make a guest post.

**Logistics**

A stickied post can be implemented by adding a field [here](https://github.com/nim-lang/nimforum/blob/master/src/frontend/threadlist.nim#L17) and adding the appropriate behavior on the front-end. A button for moderators must also be added in order to mark a post as stickied. A stickied post could also just be an ordinary tag/category (#169).

For the behavior on the front-end, there is one of several ways to implement it.

The first (and easiest) is to simply gather all stickied posts and put them at the top of the current post list. However, this would get crowded quickly and (left unsorted) could get just as crowded as the current post list. A checkbox could be added in order to view stickied posts.

The second (which requires categories #169), is to have them in a different view separate from the current post list, and sortable/filterable (#168) by category/tag. I'm in favor of this way, since then stickied posts would be more useful than just another list of unsorted (but curated) posts.