UI: close modal menus on click-away or esc. key press

[calumbell]

Currently the modal menus for source selection and reporting an issue can only be closed usinig the buttons at the bottom of the menu. I feel that we could improve the user experience by implementing widely used modal menu design patterns where the menu can also be closed by:

1. Clicking away from the modal menu (perhaps by adding an @click handler to the translucent shade that is put over the rest of the page)
2. Pressing the ESCAPE key

NB. These changes will be in addition to the current mechanisms for closing the menu. The buttons at the bottom of the modal will still work as they already do.

The <modal-dialogue> component is probably the best place to start, although the <sources-modal> and <report-issue> components might also need to be touched