Add several options to folder model

[zachmullen]

• Add compound index on (parent, name)
• Set the default sort order as +name
• Add unique constraints for sibling folder names

This depends on #24 and #23, I'll rebase this after those are merged.

[zachmullen]

Note that the reason two separate constraints are needed is because the null values are not considered equal to one another by the DBMS. See the more recent answer on this thread.

[zachmullen]

Rebase was trivial, so I'll merge when it's green.