do some validation of the KohaOpacRecentSearches cookie

do some validation of the KohaOpacRecentSearches cookie

Add validation of the value of the KohaOpacRecentSearches.  In
particular, this patch avoids the generation of an internal server
error when the OPAC is presented with an old cookie that uses the
old Storable-based serialization.

This patch also moves parsing of the cookie value into a
new routine in C4::Auth, ParseSearchHistoryCookie, and adds
a test case.

To test (in conjunction with the previous patch):

Exercise the OPAC search history functionality, after
turning on the EnableOpacSearchHistory syspref:

- As an anonymous user, conduct a variety of searches,
  including ones that include non-ASCII characters
- Check the search history and verify that all searches
  are listed
- Apply this patch and the previous one.
- Do *not* clear the KohaOpacRecentSearches cookie
- Check the search history and verify that no searches
  are listed any more
- As an anonymous user, conduct a variety of searches,
  including ones that include non-ASCII characters
- Check the search history and verify that all searches
  are listed
- Log into the OPAC
- Verify that current and past searches are listed in
  search history.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>