Clean up the database schema

[MaybeJustJames]

• "Pathway"s (as in the data describing a metabolic path from a tracer to a gauge) needs a different name because it's overloaded with "Pathway" from BioCyc (See https://github.com/vibbits/ttfd/blob/master/server/ttfd/models.py#L205).
• The Label.metabolite field can be removed (See https://github.com/vibbits/ttfd/blob/master/server/ttfd/models.py#L130). Instead the metabolite can be mapped from the Pathways table. This should free a lot of redundant space in the labels table.
• The Reaction.direction field seems to never be used (See https://github.com/vibbits/ttfd/blob/master/server/ttfd/models.py#L280). It can be removed.