[Taler] Allow history rewrite on feature branches

[Lukas Großberger]

Dear Taler team,

currently, a server side hook prevents people from force pushing (i.e.
rewriting version history).
I find amending commits or squashing commits to be quite useful to keep
the history of a feature branch tidy before merging.
How do you feel about making an exception in that hook for branches with
the prefix "feature/"?

Best, Lukas