use &times; instead of hardcoded unicode times

This very simple PR corrects a UTF-8 compatibility issue: after logging in you may get "incompatible character encodings: UTF-8 and ASCII-8BIT".  I haven't found a solution to this problem in Rails.  If there's a clear solution, please let me know.

I tracked the problem down to the hard coded UTF-8 character for times used in the pjax layout for dismissing messages.  Elsewhere in the code, the HTML entity is used, so I replaced the UTF-8 character with this.