snapshot: Update descriptions and UI options

- Mention that btrfs filesystems are only supported 'currently'.

- Clarify 'Free Space'. Explain how it actually works.

- Show '%' after the number in free space drop down. Also add 70%.

- Maintain the earlier order of showing timeline snapshots first and then
  software snapshots. This is order of importance.

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

plinth/modules/snapshot/__init__.py

@@ -42,7 +42,7 @@ description = [
     _('Snapshots are taken periodically (called timeline snapshots) and also '
       'before and after a software installation. Older snapshots will be '
       'automatically cleaned up according to the settings below.'),
-    _('Snapshots work on btrfs file systems only and on the root '
+    _('Snapshots currently work on btrfs file systems only and on the root '
       'partition only. Snapshots are not a replacement for '
       '<a href="/plinth/sys/backups">backups</a> since '
       'they can only be stored on the same partition. ')

plinth/modules/snapshot/forms.py

@@ -24,15 +24,12 @@ from django.utils.translation import ugettext_lazy as _
 
 class SnapshotForm(forms.Form):
     free_space = forms.ChoiceField(
-        label=_('Free Space'),
+        label=_('Free Disk Space to Maintain'),
         help_text=_('Maintain this percentage of free space on the disk. '
-                    'The default value is 30%.'),
-        choices=[(i / 100, i) for i in range(10, 70, 10)])
-
-    enable_software_snapshots = forms.ChoiceField(
-        label=_('Software Installation Snapshots'), help_text=_(
-            'Enable or disable snapshots before and after software '
-            'installation'), choices=[('yes', 'Enabled'), ('no', 'Disabled')])
+                    'If free space falls below this value, older snapshots '
+                    'are removed until this much free space is regained. The '
+                    'default value is 30%.'),
+        choices=[(i / 100, '{}%'.format(i)) for i in range(10, 80, 10)])
 
     enable_timeline_snapshots = forms.ChoiceField(
         label=_('Timeline Snapshots'),
@@ -40,6 +37,11 @@ class SnapshotForm(forms.Form):
                     '(hourly, daily, monthly and yearly).'),
         choices=[('yes', 'Enabled'), ('no', 'Disabled')])
 
+    enable_software_snapshots = forms.ChoiceField(
+        label=_('Software Installation Snapshots'), help_text=_(
+            'Enable or disable snapshots before and after software '
+            'installation'), choices=[('yes', 'Enabled'), ('no', 'Disabled')])
+
     hourly_limit = forms.IntegerField(
         label=_('Hourly Snapshots Limit'), min_value=0,
         help_text=_('Keep a maximum of this many hourly snapshots.'))