Add ESLint no-alert shame rule

[pdehaan]

alert() is a bit gross. So adding ESLint rule that warns on usage so we can eventually be shamed into a better UI solution (since alert() and prompt() and confirm() can eventually be dismissed by the user with a "stop doing this" option -- which would be some bad UX).

$ git grep "alert" frontend/src

frontend/src/upload.js:          alert(str);
frontend/src/upload.js:        .then(alert);

[dannycoates]

sure. i don't look at warnings anyway 😜