[base + modules] Issue #670 - Remove side effects in manage_schema functions

[robvandenbogaard]

From #670

To avoid concurrency problems in loading data models between dependent modules, remove (database) side effects from manage_schema/2 functions, because in its current form the intended use is to return the data model declaration and let Zotonic take care of installing/updating in dependency order.

[rl-king]

ping @mworrell