Improve randomness

[paragonie-scott]

Closes #5266

[alecpl]

I don't like it. There's no exceptions handling where random_* functions are used. We should just use rcube_utils::random_bytes(). From the other side, we'll not hit lack-of-entropy issue if we use real random functions everywhere?

[paragonie-scott]

In that case: You might also want to consider creating a rcube_utils::random_int() that handles exceptions from \random_int() and replace all mt_rand()/rand() invocations with rcube_utils::random_int().