Context issues

[jemrobinson]

:white_check_mark: Checklist

• [x] I have searched open and closed issues for duplicates.
• [x] This is a problem observed when deploying a Data Safe Haven.
• [x] I can reproduce this with the latest version.
• [x] I have read through the documentation.
• [x] This isn't an open-ended question (open a discussion if it is).

:computer: System information

• Operating System:
• Data Safe Haven version:

:package: Packages

List of packages

```none Paste list of packages here ```

:no_entry_sign: Describe the problem

• dsh context update --name does not update the name
• The name of a dsh context cannot have hyphens (as it's used to e.g. create a storage account name) but this is not enforced.
  • we should either enforce this when creating the file or remove forbidden characters when creating the SHM storage account.

:deciduous_tree: Log messages

Relevant log messages

```none Your log details here ```

:recycle: To reproduce