www: Bootstrap-styled buttons

Replaced buttons with Bootstrap-styled ones. While at it, I took the
opportunity to changed the color from red (which generally symbolizes
errors) to green.

Also made a small CSS change to remove the white space to the left of
the date selector at the add/edit prefix page.
www: add AVPs for pools

This adds the ability to add, modify and remove AVPs for pools in the
web UI.

Fixes #639
Merge branch '639-add-web-avps-for-pools'
Merge pull request #660 from garberg/unify_buttons

www: Bootstrap-styled buttons
Update NEWS file for Icarus - v0.27
Improve SQL upgrade speed
backend: allow configuration of forks

There seems to be a bug that hits us on certain Debian machines where we
have concurrency issues overwriting data of the different worker
processes (don't ask me how that is possible).

It is related to the forking code and while it normally is a sane
default to fork it's simply not a good idea if it's not working :P

This code allows the configuration of number of forks. If the forks
option isn't set in the configuration file, we will default to not
forking.
www: make expires not required

The expires field was required.. now it's not. It will default in the
backend to positive infinity.
Merge branch 'backend/fork-cfg'
Bump version to 0.27.1
backend: add pg contrib dependency

We need postgresql-contrib!
whoisd: remove dep on python-all
Merge branch 'backend/add-pg-contrib-dep'
Temporarily revert whoisd dependencies

We clearly need to spend some more time on this change to have whoisd
build correctly. During the meantime, I'm re-enablling the dependency on
python-all so that Travis-CI builds pass.