Make `bindata` tag the default

[silverwind]

I think most people are building gitea with bindata to generate a self-contained binary, so I would suggest adding bindata do the default tags and introduce a nobindata tag for bindata-less builds.

This will be a breaking change only for people who built without bindata, which I think is a miniscule amount.

[wxiaoguang]

You need to redesign the assets system.

At the moment, non-bindata is mainly used for development. "bindata" won't load the assets in working directory.

[silverwind]

In theory, dev could just pass nobindata. Or would that be a problem in goland etc?