This change replaces cz-conventional-changelog with cz-commitlint, which
automatically configures Commitizen using our commitlint configuration
file. Currently, we use some manual Javascript magic to load our
Commitizen configuration into commitlint (the opposite of what's
introduced by this change), which can be removed.

With this change, we also move our commitlint configuration into a
new `changelog.yaml` file. This file holds the same data as `.cz.json`
previously did.

Change-Id: I14ff2308f1a0b2b293c2128b28ca2df578ce9c1c
Signed-off-by: Chris Kay <chris.kay@arm.com>