Add 'Enter' key as a hotkey to open selected item

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.

internal/template/templates/common/layout.html

@@ -176,7 +176,7 @@
 
                 <p>{{ t "page.keyboard_shortcuts.subtitle.actions" }}</p>
                 <ul>
-                    <li>{{ t "page.keyboard_shortcuts.open_item" }} = <strong>o</strong></li>
+                    <li>{{ t "page.keyboard_shortcuts.open_item" }} = <strong>o</strong>, <strong>Enter</strong></li>
                     <li>{{ t "page.keyboard_shortcuts.open_original" }} = <strong>v</strong></li>
                     <li>{{ t "page.keyboard_shortcuts.open_original_same_window" }} = <strong>V</strong></li>
                     <li>{{ t "page.keyboard_shortcuts.open_comments" }} = <strong>c</strong></li>