This introduces support for storage backends for certificate-related

data. Backends are responsible for adding, deleting, fetching, and

iterating through SSL/TLS certificates, CA bundles, and verified

certificate fingerprints, along with providing stats on the data stored.
These can all be tied to a specific Local Site or to the global site.

Each object has an ID specific to the backend, which can be used for

lookup or deletions. IDs must be set by the backend, and consumers of

the backends must take care not to allow user-provided input to be used

without validation.
Backends are given a space within the site's data directory where they

can store files. Most libraries and tools expect a file path to PEMs, so

the backend is responsible for ensuring that those files exist when

needed.
Each object type being managed has a "stored" counterpart class. These

are subclassed by the backend and used to associate any necessary

information needed to identify or place the object somewhere in a

storage system.
By default, we ship a file-based storage backend. This is designed for

most single-server or managed-sitedir needs. All data is managed on the

filesystem, using known paths and filenames. These files can be

hand-added to the correct directory, so long as they follow the correct

naming and file format schemes.
For most standard deployments, this will more than suffice. However,

we'll be providing a database-managed solution in Power Pack, for

handling very large numbers of certificates across unlimited numbers of

Local Sites in a manner that allows for synchronizing across servers.
For Review Board 6.0.0, we may not include any UI for interfacing with

storage, but rather can provide documentation for file-based storage. UI

would come in a follow-up release.

Unit tests pass.