Fix vulnerability in config user service backend #1411
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Add unit tests for exploiting the email address or display name in the config user service by using newlines in the values.
Te `StoredUserConfig` did not handle sections without a subsection. When the subsection did not exist, i.e. was `null`, then the subsection name would be set to the string "null". This is not how the config file format works. It should create a `[SECTIONNAME]` entry instead. This fix handles a `null` subsection correctly, by handling it as a section without a subsection.
The `StoredUserConfig` only escaped the escape character, i.e. backslash. But it does not escape control characters like tab or newline. This introduces a vulnerability where an attacker can create new entries in their user account and create new accounts. In addition, other characters are also not properly handled. Field values with a comment character need to be quoted. This only happens for the `#` character and only when the value starts with it. Also the quote is note escaped in values. This change completely rewrites the `escape` method of `StoredUserConfig`. It takes care of properly escaping characters that need escaping for the git configuration file format. This fixes #1410
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