Improve a bit the code of SpecialAllpages::showChunk()

Improve a bit the code of SpecialAllpages::showChunk()

- Early return when including the page, makes one indentation
  level less for the rest of the method
- Directly put top and bottom links in an array, so that it is
  easier to see where they are going to be displayed
- Group the HTML generation for the top form at the end of the
  method again for better readability

Change-Id: I70c174a4c6363b2303cb5110782c5a3375640f2d