feat: implement maintenance page

[Agrendalath]

This implements a per-instance maintenance page. Internal teams can use it to run QA while providing learners with a meaningful status page in the meantime.

1. To enable it, set the DRYDOCK_MAINTENANCE_ENABLED variable in the Tutor config.
2. To manage allowed IP addresses, use the DRYDOCK_MAINTENANCE_ALLOWED_IPS variable.
3. You can also define a custom maintenance page with the DRYDOCK_MAINTENANCE_PAGE_DIR variable. See the README for details.

Authors' notes

1. I wondered where to implement this (Harmony module that modifies the LB, external Terraform module, another Tutor plugin, Django app, etc.), but this plugin lets site operators trigger the maintenance page individually, without applying it to the whole cluster, and makes it trivial to set up.
2. The maintenance page is intentionally limited to the LMS, CMS, and MFEs, and does not cover the IDAs (e.g., Course Discovery, Enterprise Catalog).
3. Please note that the maintenance page is for convenience only and should never be treated as a security boundary.
4. The default page is intentionally very simple. It exists mainly to provide the solution that's working out of the box - without requiring hundreds of lines of config.

Private-ref: BB-11200