Add visible label for selecting inbox comments to increase clickable area

Steps to reproduce

1. Log in

2. Find a work and leave a comment on it (or post a work)

3. Log out

4. Reply to the comment you left (or comment on your work)

5. Log in

6. Hi, username! > My Dashboard > Inbox

7. Check the box to select the comment

What happens
Because this is just a checkbox, the area to click is very small – not the entire grey button-style area around the checkbox, but just the little checkbox inside it. This is not super obvious and it's rather annoying, especially if you have certain types of mobility issues.

What should happen
To increase the area you can click, we should add a visible label to the checkbox. "Select" should be sufficient text. (Please make sure to remove the now-redundant "select this comment" title attribute from the input, and you'll also need to remove the "action" class from the surrounding li.)