Simplify progress (re)sets

This patch is a follow up to the discussion from #559 about potentially
simplifying progress interactions.

This patch essentially reduces the interactions to two simple options
which allow users to always marks books as finished or discard the
progress and thus mark them as not yet started with a single
interaction:

- If users have already listed to the book, regardless of the state of
  the progress (finished, half way through, …), they get an option to
  discard the progress. This replaces the additional “mark as not
  finished” option but it should still be clear to users what happens.

- If a book is not yet finished, regardless of the state of the progress
  (not yet started, half way through), users get a “mark as finished”
  option which sets the progress to 100%.

This is hopefully a bit simpler than what we have now but also doesn't
require a user to figure out that you need to first mark a book as
finished to be able to discard the progress.

pages/item/_id/index.vue

@@ -372,12 +372,14 @@ export default {
           })
         }
 
-        items.push({
-          text: this.userIsFinished ? 'Mark as Not Finished' : 'Mark as Finished',
-          value: 'markFinished'
-        })
+        if (!this.userIsFinished) {
+          items.push({
+            text: 'Mark as Finished',
+            value: 'markFinished'
+          })
+        }
 
-        if (this.progressPercent > 0 && !this.userIsFinished) {
+        if (this.progressPercent > 0) {
           items.push({
             text: 'Discard Progress',
             value: 'discardProgress'