Turbogears2 migration: use sqlalchemy.url iso sqlalchemy.db1.url

In Turbogears2, much of the application initialization is handled by the
framework, whereas in Pylons the application was responsible for it.
Initializing SQLAlchemy is one such part of initialization which is handled
by Turbogears2.

Turbogears2 expects the configuration file to refer to the database using
'sqlalchemy.url' rather than the current 'sqlalchemy.db1.url'. While the
exact name is not really important, not following this approach means we'll
need to override the sqlalchemy initialization method.

Therefore, as a preparation to the Turbogears2 migration, already change the
database reference string under Pylons.
When upgrading to a version of Kallithea containing this commit, the .ini
file will manually need to be adapted to remove the .db1 strings.