Add foreign key constraints to database scheme

[slodki]

Foreign key constraints will allow:

• auto-generate database schema diagrams and documentation
• detect errors in application logic implementation like moneymanagerex/moneymanagerex#1584
• enforce data consistency while modified outside application - mmdbg scripts, external SQL tools

[slodki]

I think this is a complete entity-relationship diagram for the current db schema.

Relations not implemented at SQL level:

• CUSTOMFIELDDATA.REFID (depends on CUSTOMFIELD.REFTYPE)
• ATTACHMENT.REFID (depends on ATTACHMENT.REFTYPE)
• TRANSLINK.LINKRECORDID (depends on TRANSLINK.LINKTYPE)

[slodki]

TODO: prepare upgrade script (similar to v12)

[slodki]

This will require to replace all -1 values with NULL in db...