Add external links blocking in serve

In many use cases, it is not wanted to have user accidentaly click on external links
and leave the served ZIM content.
This could be because the result is unpredictible (reader not implementing this properly)
or because the serve user knows there's no backup internet connexion or because there is
an induced cost behind external links that doesn't affect served content.

using a new flag (`blockExternalLinks`) on `Response`/`setTaskBar`, a piece of JS code
is injected into the taskbar code.
This code adds a JS handler on all link click events and verifies the destination.
If the destination appears to be an external link (1), the link target is changed to
a specific URL:

```
/external?source=<original_uri>
```

(1) external is a link that's not on the same origin and starts with either `http:` `https:` or `//`.

Server implements a new handler on `/external` that displays a new page (`captured_external.html`)
which returns a generic message explaining the situation and offering to click on the link
again should the user really want to.
This is done by specifically asking `set_taskbar` to not block external requests on that page.

This approach allows integrators using a reverse proxy to handle that endpoint differently (rebrand it)

1. `Server` now has an `m_blockExternalLinks` defaulting to `false`
1. `Server.setTaskbar` is extended to support an additional bool to set the variable.
1. `Response` now has an `m_blockExternalLinks`
1. `Response` constr expects an additional bool for `blockExternalLinks`.
1. `Response.set_taskbar` is extended to support an additional bool to set the variable.
1. JNI/Java Wrapper reflects the extensions.
1. New resource file `templates/block_external.js` (included in head_part). Should it be in skin?
1. New resource file `templates/captured_external.html` for `handle_captured_external()`
1. Added a comment on `head_part.html` to help with JS insertion at the right place
1. `introduce_taskbar()` conditionnaly inserts the JS inside the taskbar

src/wrapper/java/kiwixserver.cpp

@@ -85,6 +85,12 @@ Java_org_kiwix_kiwixlib_JNIKiwixServer_setTaskbar(JNIEnv* env, jobject obj, jboo
   SERVER->setTaskbar(withTaskbar, withLibraryButton);
 }
 
+JNIEXPORT void JNICALL
+Java_org_kiwix_kiwixlib_JNIKiwixServer_setTaskbar(JNIEnv* env, jobject obj, jboolean withTaskbar, jboolean withLibraryButton, jboolean blockExternalLinks)
+{
+  SERVER->setTaskbar(withTaskbar, withLibraryButton, blockExternalLinks);
+}
+
 JNIEXPORT jboolean JNICALL
 Java_org_kiwix_kiwixlib_JNIKiwixServer_start(JNIEnv* env, jobject obj)
 {

src/wrapper/java/org/kiwix/kiwixlib/JNIKiwixServer.java

@@ -33,6 +33,7 @@ public class JNIKiwixServer
   public native void setNbThreads(int nbTreads);
 
   public native void setTaskbar(boolean withTaskBar, boolean witLibraryButton);
+  public native void setTaskbar(boolean withTaskBar, boolean witLibraryButton, boolean blockExternalLinks);
 
   public native boolean start();