Change "Remember me" legend to "Keep me logged in", and implement it as
a customizable string.
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.
Make "Stay logged in" the default, per consensus.
Provide for situation where the string table json files are not
accessible - ie, during testing, or if the json file arrangement is
changed or misplaced.

(I wish I could determine *how* this problem slipped by, before.)
Merge pull request #351 from SpiderOak/issue-346-keeploggedin-string

Change "Remember me" legend to "Keep me logged in", and implement it as a customizable string.