Pager of Pull form uses wrong offet

Problem/Motivation

The pager of the pull form can use a wrong offset, if muliple channels with multiple pages exist.

Steps to reproduce

1. Set up at least two channels, both with more than one page.
2. Go to the second page on the first channel.
3. Change to the second channel.
4. Click "Next" to go to the second page of the channel. Now you end up on the third page.

Proposed resolution

The reason for the behaviour is, that upon visiting a specific page of a channel, the value "offset" is added to the URL. When changing the channel, an ajax request is performed and the old offset still remains in the URL (since the page does not get reloaded and the URL stays the same).

Now, when clicking "Next" to visit the second page of the channel we just changed to, the offset from the URL is read, leading to the wrong page.

To solve the issue, the offet value from the URL needs to be reset, when changing the channel (which would prevent using ajax, afaik) OR the offset value must not be read from the URL, but must be part of the form, since this allows to set the correct value in the ajax response.

There might be other solutions, I did not come up with.