Deprecate host dashboard

[Betree]

Related to https://github.com/opencollective/opencollective/issues/4863

With the introduction of the new admin panel, we should start thinking about removing the legacy host dashboard. This should be done incrementally to prevent any disruption of service.

1. [x] Add a big banner to warn fiscal hosts admins
2. [x] Change the routes. This step will mark a significant change, as many host admins may still be accessing their dashboard from the direct URL saved in a bookmark. I asked for host admins feedback in https://opencollective.slack.com/archives/CJ2ERRZQ8/p1636013493040200 to take the temperature.

• /myhost/dashboard/:section should render the admin panel with the right section selected
• /myhost/legacy-dashboard/:section should render the legacy dashboard with the right section selected

3. [ ] Remove the legacy dashboard entirely. This should only be done after a reasonable time frame (2 months?). We will remove the components and the legacy /myhost/legacy-dashboard/:section route. We will keep supporting a /myhost/dashboard/:section redirect as some places may still link there.