Changed representation of date field.
Added migration.
We only need to constrain post length on purchased jobs.
Fixed broken unit tests and broken code.
Updated help text.
Maxed out expiration for purchased jobs.
Added new models to make invoicing more static [PD-870]
Added transaction types to invoices and switched invoice creation to also create InvoiceProducts [PD-870]
Updated tests and added migrations
Reset the forms since I was going about creating invoices incorrectly
Got invoicing in a semi-working state again
Don't create the invoice until an OfflinePurchase has been redeemed
Added tests for static invoice emails
Updated invoice number in invoice emails
Merge pull request #1078 from DirectEmployers/pd-841

[PD-841] Change expiration date representation
Merge pull request #1080 from DirectEmployers/invoicing

Invoicing [PD-870]
Merge branch 'master' into basic-location-correction
Update static file, add comments, fix more brokenness