is_super_admin() usage too strict in many cases

[johnjamesjacoby]

There are a few places in bbPress where is_super_admin() checks were previously used in place of checking for Key Master status. Now that dynamic roles and caps are solid, we can reliably check for the 'keep_gate' capability.

I propose we introduce a bbp_is_user_keymaster() function, similar to is_super_admin(), to assist with this, and use it everywhere is_super_admin() is used that isn't specific to granting or revoking super admin access via the profile page.

[johnjamesjacoby]

(In [4783]) Replace is_super_admin() usages with bbp_is_user_keymaster() where appropriate. Fixes #2231.