This introduces a new reviewboard.certs module, which will be used for

all SSL/TLS certificate management support going forward.
It currently contains representations for SSL/TLS certificates + keys,

CA bundles (containing root certs and intermediary certs), and

fingerprints.
We utilize cryptography's certificate support for parsing out

information from a certificate, such as validity dates and fingerprints,

in order to avoid duplication of effort.  These are lazily-parsed as

needed. The goal is to avoid loading this information when just passing

around basic certificate information.
Going forward, the plans are to introduce a CertificateManager for

fetching/storing/deleting paths/data for certificates, and backends to

actually handle those operations. The default backend will manage this

in the data/ directory.
We'll then begin to build UI around this and tie the manager into auth,

repository, and WebHook operations, allowing people to begin formally

providing self-signed certificates and internal CA-signed certificates

into all Review Board communication.

Unit tests pass.