add settings for reader screen margin (#223)

## Summary

* **What is the goal of this PR?** 
* This PR adds a setting to control the top left and right margins of
the reader screen in 4 sizes (5, 10, 20, 40 pt?) and defaults to `SMALL`
which is equivalent to the fixed margin of 5 that was already in use
before.
* **What changes are included?**

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks, specific areas to
focus on).

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

src/activities/reader/EpubReaderActivity.cpp

@@ -16,8 +16,6 @@ namespace {
 // pagesPerRefresh now comes from SETTINGS.getRefreshFrequency()
 constexpr unsigned long skipChapterMs = 700;
 constexpr unsigned long goHomeMs = 1000;
-constexpr int topPadding = 5;
-constexpr int horizontalPadding = 5;
 constexpr int statusBarMargin = 19;
 }  // namespace
 
@@ -253,9 +251,9 @@ void EpubReaderActivity::renderScreen() {
   int orientedMarginTop, orientedMarginRight, orientedMarginBottom, orientedMarginLeft;
   renderer.getOrientedViewableTRBL(&orientedMarginTop, &orientedMarginRight, &orientedMarginBottom,
                                    &orientedMarginLeft);
-  orientedMarginTop += topPadding;
-  orientedMarginLeft += horizontalPadding;
-  orientedMarginRight += horizontalPadding;
+  orientedMarginTop += SETTINGS.screenMargin;
+  orientedMarginLeft += SETTINGS.screenMargin;
+  orientedMarginRight += SETTINGS.screenMargin;
   orientedMarginBottom += statusBarMargin;
 
   if (!section) {