Track the full paths of attachments in the Attachment object

[tagatac]

What is changing:

1. Disambiguate Filename and Filepath in the Attachment object.
2. Replace relative export paths with absolute ones.

Why this change is being made:

1. Previously, we were computing the absolute path of the attachments in multiple places in different ways depending on the options given and whether the attachment was converted or not. This was confusing and error prone (see Fix/review Exit with code 1 due to network error: ContentNotFoundError - seems to be a PNG file type issue..? #70).
2. As a result of the first change, we are using the copied path as the file to embed or reference. When embedding in PDFs, relative paths do not work, so we need to specify the copied path (under the export path in the case of the --copy-attachments option) as an absolute path.

Related issue(s): Fixes #70

Follow-up changes needed: None AFAIK

Is the change completely covered by unit tests? If not, why not?: No, similarly to converting the attachments path to an absolute path, we are not testing failures of the filepath.Abs function.