pullrequest overview: remove custom handling of 'my pullrequests'

Re-use the same displaying code for the 'my pullrequests' overview as for
the repository pullrequests overview. Remove the now unused CSS styling as
well.

This removes the 'delete' button on the author's pullrequests. One could
argue whether the pullrequest overview is the right place for this button,
while it does not appear on the pull request page itself.

my pullrequests: line up controller/template handling with repo pullrequests

Currently, the data for 'my pullrequests' is loaded dynamically through
ajax, unlike the way 'repository pullrequests' are loaded (statically).

As there is no good reason to have both treated differently, and as dynamic
loading of 'my pullrequests' is not really needed, rework the handling of
the 'my pullrequests' page with the 'repository pullrequests' page.

This includes lining up the 'show closed pull requests' checkbox/link.

This also fixes issue #102 ('my pull requests' when not logged in:
incorrect handling of login).

icon-remove: use new icon-minus-circled instead

To remove the css that set the background of the submit input (h.submit), the
element needed to be changed to <button>.