nip-13: difficulty filter

added a slider to adjust the difficulty filter, current default is
0 so all notes are rendered. increasing the filter will hide notes
with lower difficulty target.

changed the max difficulty from 256 to 32 for now so that the
range slider is usable.

this does not yet hide reactions with lower difficulty.

@@ -326,2 +326,3 @@
 function renderFeed() {
-  const sortedFeeds = textNoteList.sort(sortByCreatedAt).reverse();
+  const sortedFeeds = textNoteList
+    .filter(note => !fitlerDifficulty || note.tags.some(([tag, , commitment]) => tag === 'nonce' && commitment >= fitlerDifficulty))