oppia: Cannot Add Exploration to "Play Later" Using Keyboard

Describe the bug It is not possible to add an exploration to “Play Later” using only the keyboard.

To Reproduce Steps to reproduce the behavior:

1. Go to https://www.oppiatestserver.org/community-library
2. Hit TAB repeatedly to cycle through the explorations.

Observed behavior Two problems:

1. The button to add an exploration to “Play Later” is never selected by the TAB key
2. There is no visual cue to indicate which explorations are selected

Expected behavior It should be easy to see which exploration is selected, and it should be possible to add an exploration to “Play Later” using only the keyboard.

Possible steps to fix this issue

1. Make sure you have cloned Oppia and are able to run the dev server.
2. Navigate to core/templates/pages/learner-dashboard-page/learner-dashboard-icons.component.html
3. The button is hidden when the user isn’t hovering over the tile. Since hovering isn’t possible without a mouse, this property (and the corresponding function + any unit tests) should be removed so the button is always visible.
4. Now, to allow for the button to be selected via the keyboard, try adding the tabindex property to the appropriate element.

Desktop (please complete the following information; delete this section if the issue does not arise on desktop):

• OS: macOS
• Browser: Firefox
• Browser-version: 99.0.1