Whoops!  Rectify fouled appStrings.json by clearing out the rest of the
only partially-removed "Customization instructions" entry.

This was blocking proper load of the appStrings file, causing undetected
lack of operation.

Also, add console.log() diagnostics to customizeStrings, to indicate
when (default and custom) strings files fail to load, including failure
details.
Replace the other incidences of "Remember me" text with the "Keep me
logged in" substitution, including the one in index.html, which requires
active DOM manipulation.