Link to admin panel

### Your use case

#### What would you like to do?

Similar to the "Upgrade to Element Home" button that appears for matrix.org users, we would like to have a button that allows a customer direct access to their admin panel. If their homeserver is hosted in EMS, then the link would be to the EMS admin page. If their homeserver is hosted on-premise, then the link would be to the admin panel served up by the synapse-admin package.

#### Why would you like to do it?

We would like to do this to give our customers quicker access to their admin panels.

#### How would you like to achieve it?

By putting a button in the user menu in the same spot as the "Upgrade to Element Home" button.

### Have you considered any alternatives?

The alternative is what exists today where our customers have to keep track of these URLs independent of our application.

### Additional context

_No response_