Add sort priority field to branch model

This enables sorting the branch drop-down in a configurable manner (e.g.
so that you can have them in order from newest to oldest).

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>

layerindex/context_processors.py

@@ -11,7 +11,7 @@ def layerindex_context(request):
     if not current_branch:
         current_branch = 'master'
     return {
-        'all_branches': Branch.objects.all(),
+        'all_branches': Branch.objects.all().order_by('sort_priority'),
         'current_branch': current_branch,
         'unpublished_count': LayerItem.objects.filter(status='N').count(),
     }

layerindex/models.py

@@ -15,6 +15,7 @@ class Branch(models.Model):
     name = models.CharField(max_length=50)
     bitbake_branch = models.CharField(max_length=50)
     short_description = models.CharField(max_length=50, blank=True)
+    sort_priority = models.IntegerField(blank=True, null=True)
 
     class Meta:
         verbose_name_plural = "Branches"