Fixed issue with main menu not showing up when there is no custom navigation.

Closes #63 (closed)

src/components/page/Page.tsx

-import KeyboardArrowUpIcon from '@mui/icons-material/KeyboardArrowUp'
-import MenuIcon from '@mui/icons-material/Menu'
 import {
   Box,
   Container,
@@ -17,6 +15,7 @@ import {useScroll} from 'react-use'
 
 import {SxProps} from '../../utils/types'
 import MainMenu from '../app/MainMenu'
+import {ArrowUp, Menu} from '../icons'
 import Breadcrumbs from '../navigation/Breadcrumbs'
 import usePage, {pageContext} from './usePage'
 
@@ -53,7 +52,7 @@ function ScrollTop() {
         sx={{position: 'fixed', bottom: 16, right: 16, zIndex: 1000}}
       >
         <Fab size='small' title='Scroll to top'>
-          <KeyboardArrowUpIcon />
+          <ArrowUp />
         </Fab>
       </Box>
     </Fade>
@@ -346,7 +345,7 @@ export default function Page({
 
   return (
     <PageRoot className={classes.root} sx={sx}>
-      {navigation && showNavigationAsDrawer && variant === 'page' && (
+      {showNavigationAsDrawer && variant === 'page' && (
         <PageNavigationDrawer
           className={classes.navigationDrawer}
           anchor='left'
@@ -375,14 +374,14 @@ export default function Page({
       )}
       <PageScrollableContent fullwidth={fullwidth}>
         <PageBreadcrumbsWithNavigation>
-          {navigation && showNavigationAsDrawer && variant === 'page' && (
+          {showNavigationAsDrawer && variant === 'page' && (
             <IconButton
               data-testid='navigation-button'
               color='primary'
               size='small'
               onClick={handleOpenNavigation}
             >
-              <MenuIcon />
+              <Menu />
             </IconButton>
           )}
           <Breadcrumbs />