Add script for finishing long-lived branches

This allows finishing release, hotfix, and long-lived feature
branches. It tags the commit, does the necessary merges and deletes
the long-lived branch.

Change-Id: Idf9b99e757e4927411849022d8a0f3364e408dfe
Remove some menu items for support branches

A support branch is similar to the master branch. There shouldn't
be any direct work on a support branch. Therefore the menu items
for starting/submitting/finishing a bugfix on a support branch
were removed.

Change-Id: I6e8e9206caad5ccbde262eb5e3abf9c1b57653fd
Remove git-flow dependency

We no longer need git-flow installed since the scripts manage
all aspects. We still use some settings that git-flow used to
make.

Change-Id: I36a05527110f4781eed52b61d5fd111eea879808