date metadata is ignored

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If you specify a date in the subject's metadata 2 problems will arise
1. two dates will show up in the YAML Frontmatter (the date the post was received and later the date specified in the subject)
2. image files are saved in a date folder corresponding to the date of the post not the date specified in the metadata

_Solutions:_
Once the post is deemed to not be spam look for a specified date. If it is found, parse it into a DateTime object so that we can use it for the image folder. Add an unless around the auto-generated date key for the YAML frontmatter that skips it if the user has specified one.

No files or tests are named. Trace the post-processing path after spam detection, then inspect how the metadata date, YAML frontmatter date, and image date folder are derived; done means a specified date controls the image folder and prevents a duplicate generated frontmatter date.

Written by the indexing model from the issue text.