FR: Refactor `data_types` for timestamps to be better for filtering

[obsidianforensics]

Right now, the node.data_type for the various different timestamps is the name of the timestamp format (ex: epoch-seconds or webkit). It would be nice to standardize these a bit to enable easier searching/filtering of Unfurl output. One example would be a user Unfurling a bunch of URLs, then wanting to see any timestamps found (these would be challenging to find now). Refactoring the data_type for these to start with timestamp would give something consistent to filter on.