Use path fields instead of `path_hash`.

We are now using the `path` and `canonical_path` fields throughout the
app, instead of the hashed variants, which we needed because of MySQL
limitations.

We've flipped the index in the `host_paths` table to use the `path`
field, and declared the hash variants as nullable throughout. We're not
removing the columns yet, so that we don't break existing processes that
try to write to them; we'll do that in a subsequent migration.

We also need to index the `canonical_path` field, to replace the index
on `c14n_path_hash`.

This mostly reverts commit 52f0b9fc800a62468ee25d57fe95dec765d1fdc1.

Conflicts:

	db/schema.rb
	lib/transition/import/hits_mappings_relations.rb
	spec/lib/transition/import/hits_mappings_relations_spec.rb