string utils: use string_view

src/account.h

src/account_factory.cpp

src/account_factory.h

src/ftp_server.cpp

src/ftp_server.h

src/ip_utils.cpp

src/ip_utils.h

src/jamidht/jamiaccount.cpp

src/jamidht/jamiaccount.h

src/jamidht/namedirectory.cpp

src/jamidht/namedirectory.h

src/manager.cpp

src/manager.h

src/preferences.cpp

src/preferences.h

src/sip/sdp.cpp

src/sip/sdp.h

src/sip/sip_utils.h

src/sip/sipaccount.cpp

src/sip/sipaccount.h