filters: re-add the possibility of filtering undelegated patches

filters: re-add the possibility of filtering undelegated patches

The filters.py redesign that happened for patchwork 1.1 removed
a functionality that we use a lot: to filter patches that weren't
delegated to anyone.

Also, it is a way harder to find someone to delegate with a free
text input. Use, instead a combo-box just like before.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: f439f541 ("Add delegate filter autocomplete support")
Closes: #60
[stephenfin: Rework release note and fix some style issues]