There are a few things that need to be done, to make this work.
First, we need to register `Enter` as another hotkey that opens the
selected item.
However, by default the `KeyboardHandler` will override all default
actions. That might make sense for any other key, but for the `Enter`
key, we want to keep the default behavior (i.e. follow a selected link
or press a button). So for this single key event, we do not call
`preventDefault()`.
I see this as unproblematic for the following reasons.
1. With the changes from #2348, when we're in a list of items (articles,
categories, feeds), there is no link selected. This is what made the
`Enter` key work _implicitly_ in the past. With nothing selected, the
`Enter` key will do nothing by default.
2. If we have **any** link selected (including when we are in a view
with a list of selectable items), we'll get the default action of
`Enter` (i.e. follow a link), which is exactly what we had before.
Lastly, we need to update the list of keyboard shortcuts displayed when
pressing `?`.
This fixes#2366.
