Allow to disable automatic addon updates

[allo-]

• extensions.update.autoUpdateDefault enabled automatic addon update and should probably be disabled by default
• extensions.update.enabled enables automatic update search for addons and should probably stay enabled.

Pro:

• In chromium I already saw extensions that became adware (any maybe worse) after an update. There are some extensions, that were sold. I think there are some people who sold extensions together with the whole developer account, so the user does not know that something was changed, and then add things like ads or reflink-injection.

Contra:

• Not updating addons is a security risk

Especially since we bundle addons that are often not the latest version, it is important that the user updates them right when starting to use the new profile and in general he should check from time to time for updates and do them.