Restructure sagas

openbis_ng_ui/src/index.js

openbis_ng_ui/src/store/reducers/exceptions.js

openbis_ng_ui/src/store/reducers/loading.js

openbis_ng_ui/src/store/reducers/mode.js

openbis_ng_ui/src/store/reducers/reducer.js

openbis_ng_ui/src/store/reducers/reducer.test.js

openbis_ng_ui/src/store/reducers/root.js

openbis_ng_ui/src/store/reducers/sessionActive.js

openbis_ng_ui/src/store/sagas/page.js

openbis_ng_ui/src/store/store.js