Sanitize gadget name

MediaWikiGadgetsDefinition does some basic gadget name sanitization
and we have to do the same when checking "is gadget enabled for user"

Changes:

• sanitize gadget name same way as MediaWikiGadgetsDefinitionRepo::newFromDefinition() does.
• add try{} catch() when loading gadget as getGadget might throw an exception

Bug: T160081
Change-Id: Ia7a57e9dcfa3b25129d6d2bf75795372fad2b251