Move authentication to a separate login servlet.

Move authentication to a separate login servlet.

        * src/main/java/org/glom/web/client/OnlineGlomService.java:
        * src/main/java/org/glom/web/client/OnlineGlomServiceAsync.java:
        * src/main/java/org/glom/web/server/OnlineGlomServiceImpl.java:
Move checkAuthentication() and isAuthenticated() to:
        * src/main/java/org/glom/web/client/OnlineGlomLoginService.java
        * src/main/java/org/glom/web/client/OnlineGlomLoginServiceAsync.java
        * src/main/java/org/glom/web/server/OnlineGlomLoginServlet.java
        * src/main/webapp/WEB-INF/web.xml: Mention the new servlet.
        * src/main/java/org/glom/web/client/activity/DocumentLoginActivity.java:
        * src/main/java/org/glom/web/client/activity/HasDocumentActivity.java:
Adapted.

We already have to (but do not yet) share the login details between
the regular and images servlets, so we might as well separate it
competely. This assumes that I will find a way to share the data
between servlets.