Improve behaviour when licence limits are exceeded

At present, when the licensed number of users is exceeded, everyone is redirected to the user admin page where a message saying "You have <n> more Crucible users than allowed by your licence" is displayed. Most users will not be authenticated as the administrator, and so will just see the administration login, with no explanation.

We need to make it far easier to understand what is happening – users should be sent to a page which explains the problem, and includes a link to the user admin page.

We must also log explanatory messages.