disks: Minor changes for styling and performance

- Enable cached results when checking if the user is an admin. Cached results
  are only invalid when a user newly get admin permission or when an existing
  user loose admin permission. Here the consequence of showing wrong results is
  minor.

- Use all small case for variable names.

- When we use a utility method from Plinth, we can trust it to give expected
  values. Otherwise, we should fix the method to be more robust.

- Don't recommend the user to go to disks configuration page as there is not
  much there to do to free up space. The action might be removing apps, deleting
  downloaded content (downloaded via torrents) etc.

- df is unlikely to return '-1' for size of disk.

- Avoid using -1 as error value.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/modules/disks/views.py

@@ -76,41 +76,34 @@ def expand_partition(request, device):
 
 def warn_about_low_disk_space(request):
     """Warn about insufficient space on root partition."""
-    if not is_user_admin(request, cached=False):
+    if not is_user_admin(request):
         return
 
     disks = disks_module.get_disks()
     list_root = [disk for disk in disks if disk['mountpoint'] == '/']
-    perc_used = list_root[0]['percentage_used'] if list_root else -1
-    size_str = list_root[0]['size'] if list_root else '-1'
-    size_Bytes = _interpret_size_string(size_str)
-    free_Bytes = size_Bytes * (100 - perc_used) / 100
-    free_GiB = free_Bytes / 1024 ** 3
-    free_str = _format_bytes(free_Bytes)
-
-    if perc_used < 0 or free_GiB < 0:
-        logger.exception('Error getting information about root partition.')
+    if not list_root:
+        logger.error('Error getting information about root partition.')
         return
 
-    msg_str = format_lazy(
+    perc_used = list_root[0]['percentage_used']
+    size_bytes = _interpret_size_string(list_root[0]['size'])
+    free_bytes = size_bytes * (100 - perc_used) / 100
+
+    message = format_lazy(
         _('Warning: Low space on system partition ({percent_used}% used, '
-          '{free_space} free). Check the Disks Configuration to resolve '
-          'this problem.'),
-        percent_used=perc_used, free_space=free_str)
+          '{free_space} free).'),
+        percent_used=perc_used, free_space=_format_bytes(free_bytes))
 
-    if perc_used > 90 or free_GiB < 1:
-        messages.error(request, msg_str)
-
-    elif perc_used > 75 or free_GiB < 2:
-        messages.warning(request, msg_str)
+    free_gib = free_bytes / (1024 ** 3)
+    if perc_used > 90 or free_gib < 1:
+        messages.error(request, message)
+    elif perc_used > 75 or free_gib < 2:
+        messages.warning(request, message)
 
 
 def _interpret_size_string(size_str):
     """Convert size string to number of bytes."""
-    if size_str is None or not size_str:
-        return -1
-
-    if size_str[-1] in '-10123456789':
+    if size_str[-1] in '0123456789':
         return float(size_str[:-1])
 
     if size_str[-1] == 'K':