[IMP] mail: introduce and clean follower views

...l/static/src/components/chatter_topbar/chatter_topbar.xml

addons/mail/static/src/components/follower/follower.js

addons/mail/static/src/components/follower/follower.xml

...c/src/components/follower_list_menu/follower_list_menu.js

.../src/components/follower_list_menu/follower_list_menu.xml

addons/mail/static/src/models/follower.js

addons/mail/static/src/models/follower_list_menu_view.js

addons/mail/static/src/models/follower_view.js

addons/mail/static/tests/helpers/mock_server.js

...atic/tests/qunit_suite_tests/components/follower_tests.js