Fix XML parsing failures due to non-breaking spaces

Using `&nbsp;` for non-breaking spaces is not supported in the XML
serialization of HTML5, so replace with the character code equivalent so that
SimpleXML can properly parse the page.
Set guzzle user agent in config
Iterate through issue pages with pager-next to find total page count

pager-last no longer exists on issue pages, so we need to scan every page to
find the end
Store a null count if parsing XML fails, instead of killing entire run