Improve the image selector in Attachment Selector

Issue reported by alexandra.ifrim

Currently the selected image has a different background and lacks the 'tick' icon. Users are expected to click on the 'tick' icon in order to select the new image.
The current behaviour creates confusions because:

• 'tick' icon implies that the image is selected, not that is an activator in order to select it;
• the selection background-color being gray is less visible than the white background, so the user doesn't notice the current selection;
• the 'tick' clickable area is very small and prone to error;
• users will rather click on the big preview, instead of the 'tick' icon in order to select the new image; Clicking on the preview opens the image in browser, instead of selecting. Going back loses the context.

Improvements requested:

• Replace the behaviour with clicking on the 'tick' with clicking on the image in order to select;
• Improve the background-color selection
• Rename the 'Cancel and return to page' to 'Cancel'