feat: Improve UX to match new mockups

Uses new API features for preview, deleting, pinning, and closing. Refactors and improves styling to match new mockups.
Adds the new post filter bar element.

src/discussions/posts/data/selectors.js

@@ -37,12 +37,6 @@ export const selectAllThreads = createSelector(
 
 export const threadsLoadingStatus = () => state => state.threads.status;
 
-// TODO: eventually this should be server-side filtering
-export const selectUserThreads = author => createSelector(
-  [selectAllThreads],
-  threads => threads.filter(thread => thread.author === author),
-);
-
 export const selectThreadSorting = () => state => state.threads.sortedBy;
 
 export const selectThreadFilters = () => state => state.threads.filters;