add indication that login/signup request is in progress

when you click login or sign up, there should be some progress indication.

we could do a spinner, or we could change the cursor.

to change the cursor, we would put this in the css:

html.busy, html.busy * {  
  cursor: wait !important;  
}

and then add the class .busy to html when the xhr request is first fired off, then remove the class if there is an error (success redirects, so no need to remove it in that case).

this works nice, i added this to crabgrass in a few places. but there was this weird problem with chrome where the cursor would only update if you moved it slightly, but would stay with the old cursor if the mouse was perfectly still. i can't remember if i figured out a fix to this or not.

(from redmine: created on 2013-07-11, closed on 2013-09-27)