make default reader settings changeable (#217)

* [#214] Move "reader settings" util functions into new file

* [#214] Extract "options" of "ReaderSettings" to new component

* [#214] Add component for changing the default reader settings

* [#214] Add default "ReaderSettings" to "Settings"

* [#214] Use default "ReaderSettings" stored on the server

Wait for both "manga" and "defaultSettings" to be loaded.
Otherwise, by the time the "manga" response was received, the "defaultSettings" might be still loading and thus, the local fallback "default" settings will get used as the "default settings"

* [#214] Save and update "ReaderSettings" options in case they are missing

- Manga gets read the first time => save settings in metadata
- Default settings get opened the first time => save settings in global metadata

src/App.tsx

@@ -24,6 +24,7 @@ import Settings from 'screens/Settings';
 import About from 'screens/settings/About';
 import Backup from 'screens/settings/Backup';
 import Categories from 'screens/settings/Categories';
+import DefaultReaderSettings from 'screens/settings/DefaultReaderSettings';
 import SourceConfigure from 'screens/SourceConfigure';
 import SourceMangas from 'screens/SourceMangas';
 import Sources from 'screens/Sources';
@@ -60,6 +61,9 @@ const App: React.FC = () => (
                 <Route path="/settings/categories">
                     <Categories />
                 </Route>
+                <Route path="/settings/defaultReaderSettings">
+                    <DefaultReaderSettings />
+                </Route>
                 <Route path="/settings/backup">
                     <Backup />
                 </Route>