UrlEncode the content_id.

The HumanReadableId can contains special char (`&`/`=`/...)
As it is used as to create a url in the opds template,
we must url encode it.

- We don't need to encode the book id as it is a uuid, it never contains
special char.
- We don't need to encode the book url as it is read from the library and
the url must already be correctly encoded in the library.xml.
(tests modified accordingly)

test/meson.build

@@ -29,6 +29,7 @@ if gtest_dep.found() and not meson.is_cross_build()
     data_files = [
       'example.zim',
       'zimfile.zim',
+      'zimfile&other.zim',
       'corner_cases.zim',
       'library.xml'
     ]