server: Use include directives for nginx to create an upgrade path

Created by: keegancsmith

We currently generate the nginx.conf for users, but then it is controlled by users. If we want to change a setting in nginx.conf (such as changing a proxy timeout), we then have to rely on user instructions to update the config.

Instead we can hook into the config via well placed include directives.

Also included in this PR is using go-bindata to embed config into server. This is to simplify the addition of the extra configs.

Fixes https://github.com/sourcegraph/sourcegraph/issues/1901