Refactor Hash interfaces and centralize hash function. This will allow
easier introduction of different hash function later on.
This forms the "no-op" part of the SHA256 enablement patch.
The JSONRedirect/JSONOK/JSONError functions were put into "Base" context
incorrectly, it would cause abuse.
Actually, they are for "web context" only, so, move them to the correct
place.
And by the way, use them to simplify old code: +75 -196
There are too many files under `routers/web/repo` and the file
`routers/web/repo/setting.go` is too big.
This PR move all setting related routers' body functions under
`routers/web/repo/setting` and also split `routers/web/repo/setting.go`
