Config options for Element builds

### Your use case

Each year for FOSDEM we hard code some changes (at least 3 days a year) to make the product (deployed at chat.fosdem.org) more appropriate for the audience. These changes are:
- Adding a note to the Signup/Signin page (with the mountains)
- Automatically disabling the Join/Leave events shown in timeline toggle in Settings
- Changing the HS and Search defaults (and ecluding previous year's spaces from results)
- ~~Adding a feedback label to the GHs created via the Feedback form so we know the client info and can filter~~ Be able to specify the feedback URL in config, and ideally be able to auto-fill app version, and other details

### Have you considered any alternatives?

Patching the app, but we don't really want to do that as it makes updating more awkward and we end up dropping the customisations for most of the year, which means our users don't get a great UX.

### Additional context

[Changes are here](https://github.com/matrix-org/matrix-react-sdk/compare/develop...jaywink/fosdem)