AddtionalInformationForm and RTF upload

Ability to upload RTF files from the newly created AddtionalInformationForm

  - RTF attachment upload
  - Attachment deletion
  - Attachment replacement
  - Claim#miscellaneous information is set to nil when
    AddtionalInformationForm#has_miscellaneous information is
    set to false

Undefined behaviour:
  - What happens when a non-RTF file is uploaded
Validate content-type of uploaded RTF files

Only RTF or plain text files are permitted to be uploaded.
Merge pull request #128 from ministryofjustice/rtf-upload

AddtionalInformationForm and RTF upload