Migrate menus to the CMS

[marcoacierno]

We should migrate all of our FE menus to Wagtail so we don't have any website/cms things left in our BE. Wagtail menus could be an option: https://wagtailmenus.readthedocs.io/en/stable/ but open to better alternatives

We currently have 3 menus that need to be migrated: conference-nav, program-nav and footer.

As part of this work we should also rename the handles of the menus as they are very confusing.

1. Program nav is the top links
2. Conference nav is the middle bar
3. "Cambia lingua"/"Change language" is currently hardcoded in the header; should we also consider moving it to the CMS?

We also have actions, i.e. the main buttons profile/tickets

Actions are dynamic:

1. If the conference is live we automatically switch to show the live streaming page.
2. If the user is logged in we show "Profile" or if not we show "Login"

https://github.com/pythonitalia/pycon/blob/9a17b2f1ba6b5a9ba18e8bcb4fea2021dcc78ccb/frontend/src/components/header/index.tsx#L45