Remove special character limitations in raw, masked variables

Problem to solve

GitLab is unable to mask variables with special characters.

Proposal

Allow users to mask raw variables with special characters. Since raw variable values are not expanded to check for the presence of other variables within them, they should be safe to mask when including special characters.

Use a feature flag

Obviously some characters MAY be dangerous to mask if they are special in the shell or in the backend code. Care must be taken to ensure masking is always safe, and no unsupported input chars can crash or corrupt. For this reason, this feature should be implemented under a feature flag.