A very large number of the logger calls are of the form log.Function(fmt.Sprintf(...)).
Rather than sprinkling fmt.Sprintf at every logger call site, provide formatting versions
of the logger functions and call these directly with the format and arguments.
While here remove some unnecessary trailing newlines and calls to String/Error.
Splits out the old `Errors` slice into a public `Error` string and a `InternalErrors` slice. Also removes a number of occurrences of calling `logEvent.AddError` then immediately calling `wfe.sendError` with either the same internal error which caused the same error to be logged twice or no error which is slightly redundant as `wfe.sendError` calls `logEvent.AddError` internally.
Fixes#3664.
* Move probs.go to web.
* Move probs_test.go
* Factor out probs.go from wfe
* Move context.go
* Extract context.go into web package.
* Add a constructor for TopHandler.
Joel Sing