Updated utility function for setting security level
Updated utility function for setting security level

- added _tbb_set_pref() to set firefox preferences directly instead of via UI
- replaced actionchains calls with _tbb_set_pref() in set_tbb_securitylevel()

conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop