Make hardcoded fields translatable

In Admin->Networks->[network name]->Ports->Create Port, the Device ID and
Device owner have untranslated help_text values, which get displayed as
tool tips in the UI.  This change adds the translation function to those
pieces of text.  Also fixing the Edit Port parts as well.

Change-Id: Iaddd58b50cda818e332f8bead5a02b07916a2f58
Closes-Bug: #1271348
Replace mimetype with content_type argument of HttpResponse

One occurrence of mimetype remained in the code base. Replace it with
content_type, since mimetype is getting deprecated.

Change-Id: I1bac546bdf41d785deebe3c3df6a9601575983dd
Partially-impelements: blueprint django-1point6
Merge "Make hardcoded fields translatable"