Enable declarative projects.

This allows fully declarative project specifications. This is best
illustrated by example:

* I create a new project, setting the declarative spec file to
  "spec.json" and the declarative input to a git repo pointing
  at git://github.com/shlevy/declarative-hydra-example.git
* hydra creates a special ".jobsets" jobset alongside the project
* Just before evaluating the ".jobsets" jobset, hydra fetches
  declarative-hydra-example.git, reads spec.json as a jobset spec,
  and updates the jobset's configuration accordingly:
{
    "enabled": 1,
    "hidden": false,
    "description": "Jobsets",
    "nixexprinput": "src",
    "nixexprpath": "default.nix",
    "checkinterval": 300,
    "schedulingshares": 100,
    "enableemail": false,
    "emailoverride": "",
    "keepnr": 3,
    "inputs": {
        "src": { "type": "git", "value": "git://github.com/shlevy/declarative-hydra-example.git", "emailresponsible": false },
        "nixpkgs": { "type": "git", "value": "git://github.com/NixOS/nixpkgs.git release-16.03", "emailresponsible": false }
    }
}
* When the "jobsets" job of the ".jobsets" jobset completes, hydra
  reads its output as a JSON representation of a dictionary of
  jobset specs and creates a jobset named "master" configured
  accordingly (In this example, this is the same configuration as
  .jobsets itself, except using release.nix instead of default.nix):
{
    "enabled": 1,
    "hidden": false,
    "description": "js",
    "nixexprinput": "src",
    "nixexprpath": "release.nix",
    "checkinterval": 300,
    "schedulingshares": 100,
    "enableemail": false,
    "emailoverride": "",
    "keepnr": 3,
    "inputs": {
        "src": { "type": "git", "value": "git://github.com/shlevy/declarative-hydra-example.git", "emailresponsible": false },
        "nixpkgs": { "type": "git", "value": "git://github.com/NixOS/nixpkgs.git release-16.03", "emailresponsible": false }
    }
}

src/sql/hydra.sql

@@ -30,6 +30,9 @@ create table Projects (
     hidden        integer not null default 0,
     owner         text not null,
     homepage      text, -- URL for the project
+    declfile      text, -- File containing declarative jobset specification
+    decltype      text, -- Type of the input containing declarative jobset specification
+    declvalue     text, -- Value of the input containing declarative jobset specification
     foreign key   (owner) references Users(userName) on update cascade
 );
 

src/sql/upgrade-48.sql

@@ -0,0 +1,4 @@
+-- Add declarative fields to Projects
+alter table Projects add column declfile text;
+alter table Projects add column decltype text;
+alter table Projects add column declvalue text;