[Enhancement] Pressing the "Enter" key while having a preview shot of a full page or visible area should download it

[Softvision-AndreeaCupsa]

[Affected Versions]: -Firefox 63.0.3 and up

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Steps to reproduce]:

1. Open the Firefox browser and navigate to any website.
2. Select the "Take a screenshot" option from the context menu.
3. Click the "Save full page" button.
4. Press the "Enter" key and observe the behavior of the page.

[Expected result]:

• The shot is uploaded and its page is displayed.

[Actual result]:

• Nothing happens.

[Notes]:

• When pressing the "Enter" key after making a manual selection of a screenshot, the shot is uploaded.
• The same behavior should also be implemented in Download-Only Mode.
• This issue is reproducible with the production, stage and dev version of "Screenshots".
• Attached a screen recording of the issue:

[ianb]

If we autofocus the primary button, then this should work. I'm not sure if the autofocus attribute will work here, since the page isn't being "loaded".

[clouserw]

We're getting rid of uploading, but having a default for Enter makes sense, so we'll leave this open.