Port MIME message parsing to nom

[indiv0]

Currently MIME message parsing is done by reading the file to a String and performing String-based operations. This isn't really a viable approach as a MIME message is not guaranteed to be UTF-8 compatible. Additionally, it is better to standardize our parsers on a single approach, rather than several throughout the codebase.

As such, the MIME message parsing needs to be re-written with nom.