[Validator] Added ISSN Validator
merged branch ajgarlag/feature/issn-validator (PR #7756)

This PR was squashed before being merged into the master branch (closes #7756).

Discussion
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[Validator] Added ISSN Validator

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

This PR complements the ISBN validator added some months ago to allow validation of relevant library systems data.

Commits
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77f62ec [Validator] Added ISSN Validator
[Validator] fixed CS, updated CHANGELOG