Rename variable: `fixtures` -> `fx`
Implement Program#changeIssuer

This changeset implements the new model instance method
`Program#changeIssuer`. The method ensures that the `programs` array is
modified for new issuer (adding the program) and the old
issuer (removing the program) while also updating the `issuer` field on
itself.
Implement `Issuer#removeProgram`

This simplifies `Program#changeIssuer` a bit by shifting some of the
issuer-specific logic to the issuer model, where it belongs.
Add frontend for changing program issuers.

I put a simple dropdown list on the "edit program" page to let a user
change the issuer associated with the program. The backend will
conditionally call `program.changeIssuerAndSave` instead of
`program.save` if the form submitted issuer doesn't match the stored
issuer.
Rename `Program#changeIssuer` method

...to `Program#changeIssuerAndSave`, which is more accurate to what it
does. I had an alias before, but I removed it in favor of changing the
real name of the function for the sake of clarity (in spite of brevity)
Merge remote-tracking branch 'upstream/250-change-program-organization' into v2.0
adds ability to move programs between issuers
Add security headers.
Merge pull request #269 from mozilla/security-headers

Add additional headers.
Adding additonal headers