Disallow current_user/1 predicate in submit type rules

Submit type is used as part of the mergeability bit computation, which
is persistent and so needs a consistent view of submit type. An
informal poll of repo-discuss[1] indicated that current_user/1 is
pretty much only useful for submit rules, not submit type rules.

[1] https://groups.google.com/d/topic/repo-discuss/vW6XhUOkqik/discussion

Change-Id: Ia938a685409025b36e16979fcafd92afbed2d91f
3 files changed