Display a performance warning if more than 10 roles are enabled

Problem/Motivation

Authcache Enum computes all authcache keys which are theoretically possible using a brute-force method (cartesian product). This may lead to severe performance degradation if more than ~10 roles are enabled. As a result, certain operations like saving content can get very slow if the Cache Expiration module is also in use.

Proposed resolution

Add requirements check.

Remaining tasks

User interface changes

API changes