Rename/Refactor store methods for attachments and images

[asuresh-code]

Considering @joelvdavies's comment here #91 (comment), my suggested changes are as follows:

• Rename create_presigned_get method in stores/attachment.py to create_presigned_download_url. Download URL to me implies that it would be a GET.
• Rename create_presigned_get in stores/images.py to something like create_presigned_view_url. View URL to me implies that it would be a GET.
• Create a new method called create_presegined_download_url in stores/images.py that will generate a presigned URL for downloading images. Download URL to me implies that it would be a GET.
• Rename create_presigned_post methods in stores/attachment.py and stores/images.py to create_presigned_upload_url. Upload URL to me implies that it would be a POST.

Let me know your thoughts @joelvdavies, @rowan04, @asuresh-code. This can be addressed separately.

Originally posted by @VKTB in #91 (review) (and #94 (comment))