cleanup and workflow improvements to login and JWT handling

...main/java/net/jami/jams/server/servlets/LoginServlet.java

...ami/jams/server/servlets/api/admin/users/UserServlet.java

...ver/servlets/api/install/CreateServerSettingsServlet.java

...jams/server/servlets/api/install/StartInstallServlet.java

jams-server/src/main/resources/webapp/js/api.js

jams-server/src/main/resources/webapp/js/auth.js

...-server/src/main/resources/webapp/js/server-parameters.js

jams-server/src/main/resources/webapp/js/signup.js

jams-server/src/main/resources/webapp/js/user.js

jams-server/src/main/resources/webapp/templates/search.html