Allow configuring the application from the Administration UI

Problem

The application's configuration lives on ReleaseNotes.Code.ReleaseNotesConfig and holds the instance-level defaults used when creating new release notes (default product name and release note template reference). It's however very hard for an administrator to reach:

• the page is hidden (<hidden>true</hidden>), so it doesn't appear in any navigation tree
• nothing links to it: there's no entry point from the application home page, nor from the wiki administration
• it's not registered in the Administration UI, so the only way to configure the application is to know the page name by heart, type it in the URL bar and edit the xobject by hand (which also requires an Advanced user profile)

RN-71 removed the "Force editing" step, but the page remains just as hard to find.

Proposal

Register the configuration in the wiki Administration by adding an XWiki.ConfigurableClass xobject to ReleaseNotes.Code.ReleaseNotesConfig, so that administrators configure the application from Administration > Release Notes with the configuration form displayed inline, and never have to navigate to a Code page at all:

<object>
  <className>XWiki.ConfigurableClass</className>
  <property><displayInSection>releasenotes</displayInSection></property>
  <property><configurationClass>ReleaseNotes.Code.ReleaseNotesConfigClass</configurationClass></property>
  <property><scope>WIKI</scope></property>
</object>

Notes:

• The scope property exists since XWiki 13.7 and the application requires 14.10+, so there's no need for the older configureGlobally property.
• The displayInSection value is used as a translation key prefixed with admin., so the section title should go into the ReleaseNotes.Code.Translations bundle rather than being hardcoded.
• heading can be left empty since a single ConfigurableClass object is enough here.
• This builds on RN-71: now that the page is declared with the configuration XAR entry type, saving from the Administration UI doesn't hit the extension page protection.

In addition, display a "Configure" link on the application home page for users having admin right, so that the configuration is also discoverable from the application itself.

See Making your application configurable with ConfigurableClass.