5.1 maintenance period

With the 5.0 code freeze starting on 2020-09-04T04:00:00Z, it is important to let people know what the TSC planned.

As usual, please go through old issues that are assigned to you and close those that are no longer valid with appropriate message. Also, it is a good time to look at the unreliable tests and see if you can make them more reliable.

The maintenance period will focus on providing the ability to turn off mantidplot in the build. The goal is to have the ability to turn off building mantidplot. Removing MantidPlot will be a separate issue. Until that is ready, people that are not involved are encouraged to work on outstanding maintenance that they are aware of. Once it is finished, the whole dev team should try building without mantidplot to see that the work was done correctly.