Fix agenda export of topics with nested lists

[bspekker]

Describe the bug
If a topic contains a nested list, only the first level of the list will be exported.

How to Reproduce

1. create a topic with a nested list (see example below)
2. export the topic
3. compare the exported list with the list in OpenSlides

source code of a text that will create this error:

<ul>
	<li>a<ul>
	<li>a.a</li>
	<li>a.b</li>
</ul>
</li>
<li>b</li>
</ul>

View in OpenSlides:

Export:

Expected behavior
Nested lists in topics should be fully exported. Should work for both ordered and unordered lists.

Additional context
Exports of nested lists work correctly in motions.