rename attach/[code] url to enrollmentcode/[code]

[ChristopherChudzicki]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/8742

Description (What does it do?)

Changes the enrollment code URL to /enrollmentcode/[code] and adds a redirect for it.

How can this be tested?

1. Prerequisites:
   • MITxOnline and Learn integrated locally
   • A non-sso b2b contract set up locally (List contracts with ./manage.py b2b_list contracts; see https://github.com/mitodl/mitxonline/blob/main/docs/source/configuration/b2b.rst for more.
2. Get an enrollment code for your contract in mitxonline via ./manage.py b2b_list contracts --codes <contract ID>
3. As a logged-in user not in the org yet, or as an anon user if you want to create an account, visit /enrollmentcode/[uuid]; you should be redirected to your org dashboard.
4. Using a real or fake enrollment code, visit /attach/[some-real-or-fake-code]. You should be redirected to /enrollmentcode/[real-or-fake-code].

[jonkafton]

Looks good. The Next.js docs suggests to use a permanent flag. This results in a 308, though 301 is fine for our purposes as a front end GET route (any other reason?).

Fastly should cache the 301 in the absence of any header instruction not to, though the Next.js config won't let us set an explicit max-age unfortunately.