move SimpleBlock to schema.org rdfa, add rdfa for StringBlock

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no (except for consumers of the rdfa data)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT

similar to https://github.com/symfony-cmf/ContentBundle/pull/69 but here we use WebPageElement