pyflakes jobs for some mediawiki extensions

Triggers:

mwext-BlameMaps-pyflakes
mwext-ConfirmEdit-pyflakes
mwext-DumpHTML-pyflakes
mwext-EventLogging-pyflakes
mwext-Offline-pyflakes
mwext-OpenStackManager-pyflakes
mwext-SemanticBundle-pyflakes
mwext-SemanticResultFormats-pyflakes
mwext-WikimediaMaintenance-pyflakes

Change-Id: Id378e258e4aa65fa457c5c169f8240f0e71c8a97