models: move filepath to its own model

nonstop/models.py

 from django.db import models
 from django.utils.translation import ugettext_lazy as _
 
+
+class NonstopFile(models.Model):
+    filepath = models.CharField(_('Filepath'), max_length=255)
+    creation_timestamp = models.DateTimeField(auto_now_add=True, null=True)
+
+
 class SomaLogLine(models.Model):
     class Meta:
         verbose_name = _('Soma log line')
         verbose_name_plural = _('Soma log lines')
         ordering = ['play_timestamp']
 
-    filepath = models.CharField(_('Filepath'), max_length=255)
+    filepath = models.ForeignKey(NonstopFile)
     play_timestamp = models.DateTimeField()