Add the login page in Server distribution and handle redirection depending on installed UI packages

Principle

We now have an "agnostic" login page inside the nuxeo-server distribution.
It handles form authentication and redirection logic depending on the UI package(s) installed:

• No packages: redirect to home.html.
• nuxeo-jsf-ui: redirect to the JSF UI (nxstartup.faces).
• nuxeo-web-ui: redirect to the new Web UI (ui/).
• nuxeo-jsf-ui and nuxeo-web-ui: redirect to the new Web UI (ui/). This behavior can be overridden to force redirection to the JSF UI, see below.

Implementation

To achieve this we mostly needed to:

• Move the login related resources from bundles embedded in the nuxeo-jsf-ui package to the nuxeo-platform-web-common bundle included in the nuxeo-server distribution.
• Make login.jsp post to a StartupPageServlet handling the startup page logic, so action="nxstartup.faces" became action="startup".

Note that the workaround for NXP-20013 was removed.

The startup page logic is configured by the "loginScreen" extension point, to which each UI package contributes:

nuxeo-jsf-ui:

<extension target="org.nuxeo.ecm.platform.ui.web.auth.service.PluggableAuthenticationService" point="loginScreen">
  <loginScreenConfig>
    <startupPages>
      <startupPage id="jsf" priority="10">
        <path>nxstartup.faces</path>
      </startupPage>
    </startupPages>
  </loginScreenConfig>
</extension>

nuxeo-web-ui

<extension target="org.nuxeo.ecm.platform.ui.web.auth.service.PluggableAuthenticationService" point="loginScreen">
  <loginScreenConfig>
    <startupPages>
      <startupPage id="web" priority="100">
        <path>ui/</path>
      </startupPage>
    </startupPages>
  </loginScreenConfig>
</extension>

The startupPage contribution with the highest priority wins, so "web" by default.
This can be overridden by:

<extension target="org.nuxeo.ecm.platform.ui.web.auth.service.PluggableAuthenticationService" point="loginScreen">
  <loginScreenConfig>
    <startupPages>
      <startupPage id="jsf" order="1000" />
    </startupPages>
  </loginScreenConfig>
</extension>

If there are no startupPage contributions Nuxeo redirects to the static page home.html.

See related documentation: https://doc.nuxeo.com/x/Y4rjAQ.