Replace "edX Studio" with "{studio_name}"

For [OPEN-423](https://openedx.atlassian.net/browse/OPEN-423). This probably needs further discussion and review -- do we want to have a `{studio_name}` for "edX Studio" and a `{studio_short_name}` for "Studio"? Seems like several places use "Studio" rather than "edX Studio" rather intentionally.
`# If you change this in another settings file you might have to change STUDIO_NAME also`