Display persistent message while masquerading

Problem/Motivation

It would be useful to provide the option of displaying a persistent message to make it clear to users that they are currently masquerading as another user.

Depending on the theme, the Unmasquerade link may not be very obvious, and users may forget that they are masquerading.

Proposed resolution

Add configuration options for the masquerade block:
1. display "masquerading as" message
2. display masquerade form

Change the display of the block based on these options

Remaining tasks

User interface changes

API changes

Data model changes