Tails pythonlib doesn't clean branch_name enough to fit job's name

While deploying #8658 (closed), a branch in the Tails git repo had the + character in its name, resulting in Jenkins poorly handling the automatically created job.

The Tails pythonlib had to be patched (see b4032d07) in emergency to get the automated builds deployed, but this proved that this part of its code is clearly not reliable enough.

We should probably document in contribute/git that branches should only contain characters in the [a-zA-Z0-9-_]+ range, but also implement a more elegant whitelist based method for the branch_name cleaning code.

Parent Task: #5324 (closed)

Original created by @bertagaz on 9740 (Redmine)