WebSearch: allow some guest alert/basket teasers

• When users can register accounts in some way, then display alert/basket teasers even for guests, as a publicity for these tools, inciting to create accounts. Do not print teasers only when guests are not offered info on how to register accounts (when CFG_ACCESS_CONTROL_LEVEL_ACCOUNTS>4), or when logged-in users are explicitly forbidden to use these facilities.

• Added regression test cases for the above situations.