Add Letter template cleaning tool as management command

[aj-he]

The docx files used for letters can have the field tags broken by the style runs when the file_template view processes them. This is because the style runs can split the tag name, and stop the find and replace process from identifying them.

HE has a script to fix this, but it should be added as a management command to allow this to be done more easily.

This should be added to the readme.md in the repo so that users know that they need to run this whenever they update the letters.

[aj-he]

Being worked on by @RobOatesHistoricEngland