Add a plugin for generating backend interaction URIs

One of the big frustrations we had during the migration was a nice, abstract way of showing interaction URIs - that is, URIs for interacting with the repository via a backend client app, not for webviewer URIs. Seems to me that we ought to be able to provide plugins that, while obviously being backend-specific in the URIs they generate and the underlying data they pull from, still conform to the same client-facing interface in terms of actually providing these links.

It should work something like this: an outer plugin class which takes care of grabbing settings & raw data necessary to actually figure out what all the URIs looks like. This class then produces one VersioncontrolProtocolInteractionURI object per protocol (e.g. one for git://, one for ssh://, etc.). That object's interface is responsible for reporting (at least) three things:

• A copy/pasteable raw URI
• The human-readable name of the protocol
• Whether the URI is rw or ro, & anon or authed for each (probably best expressed via bitmask)

Client code can then construct their interfaces using this information however they'd like, and can do it in a way that actually ought to work cross-vcs.