Start to handle information about Git branches

Add some new pages /branches and /branch/... as well as a new git_branches
table. Also extend the email processing to enter the branch information in to
the database.

sqitch/deploy/git_branches.sql

@@ -0,0 +1,13 @@
+-- Deploy guix-data-service:git_branches to pg
+
+BEGIN;
+
+CREATE TABLE git_branches (
+    name character varying NOT NULL,
+    commit character varying,
+    git_repository_id integer NOT NULL,
+    datetime timestamp without time zone NOT NULL,
+    CONSTRAINT name_commit PRIMARY KEY(name, commit)
+);
+
+COMMIT;