Rework recipe storage

[cmlenz]

The current storage of build recipes in the repository has the serious disadvantage that it tightly couples the recipe to the code base. For example, if Bitten is added to the environment of a project long after the project has started, the recipe won't even exist in the repository for earlier versions. Still, the commands in the recipe might be perfectly valid for older revisions.

Recipes should probably be stored in the database, or in the environment directory. When a build configuration is created, you'd need to create/select the recipe that should be used for builds from that configuration.

On build initiation, the master would send the recipe commands as the body of the <build> message (see Master Slave Protocol).

[cmlenz]

Implemented in [155] (with follow-up fixes in [156] and [157]).

Build recipes are now stored in the database. Ideally, there'd be a real user interface to edit them. For the time being, only editing the raw XML in a <textarea> is provided.