Symlink external data location

[DanNixon]

Create a symbolic link from the default external data location (as the abc user) to the /mantid_data volume.

To test:

• Invoke contains using both named Docker volumes and host filesystem mapping for the /mantid_data volume
• See that container start up time is not largely affected
• See that external data still works (just running the download test data jobs is probably enough)

(I won't publish test images for this PR as only the last layer changes so having already downloaded the latest development images means the build time will be very short)

[DanNixon]

Not overly important now that we have the configure.sh scripts.