Motion Control: Lock certain variables from users and instrument scientists: Implementation

As a member of the motion control support group, I would like certain motion control variables to be inaccessible by instrument scientists.

In the first instance, this should apply to motion control, who want to limit the access to motor parameters. The VEL, OFF and soft limits should be accessible to users. Other fields will be in the advanced motion control screen (OPI); the screen will be viewable by all users; only those who know the relevant password will be able to modify paramters on the advanced motion control screen.

#### Acceptance criteria:
1. As a user or scientist I can control the position, VEL, OFF and soft limits.
1. The advanced motion control screen is password protected:
1. As a user or scientist (who does not know the password), I can view, but not modify, parameters on the advanced motion control screen.
1. The system displays an appropriate message on the advanced motion control screen telling me why I cannot change them and who to contact should I wish to have these parameters changed.
1. As a member of the motion control team (who does know the password), I have full access to all the motor parameters on the advanced motion control screen.

#### Notes:
1. This is a follow up ticket from #2907, please see that ticket for more background about this change
1. To be clear: the request for this feature has come from the motion control group. The rationale behind protecting the advanced motion control screen, is that some of the advanced motor parameters may have safety implications (both personnel safety and machine safety). For this reason, the motion control group wishes to restrict access to these parameters. We should make this clear in the message to the users (the wording of the message will be agreed with the motion control group).
1. The motion control group will consult with scientists prior to this ticket being implemented.
1. This feature will probably operate in a manner similar system to the existing "manager mode" system
1. the password will be known only to the motion control team (and the Experiment Controls team)
1. if a scientist wishes to change an advanced motion control parameter, he should contact the motion control team
1. We will need to come up with an appropriate password, which is (relatively) easy for members of the motion control group to remember.
1. we can always change the password in future (e.g. if people forget it).
1. over time, it is quite possible that the password becomes known outside the motion control group, so again, we can change it (if necessary).