Custom separator also used for string concatenation

Problem/Motivation

In #3067803: Use custom delimiter instead of commas for multiple values the configurable separator/delimiter for text was introduced. This allows splitting up string values based on an arbitrary string. The same separator is currently used to join values together if the final value is a string.

This means, that we can't separate values in metatags fields by a special string and still produce "nice" results in html.
With a separator of "||" a taxonomy reference token would result in <meta name="NAME" content="term1||term2||term3">. This should be <meta name="NAME" content="term1,term2,term3">

Proposed resolution

Additional, separate, configurable combinator string used to combine results into a final string falling back to defaultSeparator value if not set.

Remaining tasks

- Review the proposed code
- Improve the test coverage

User interface changes

- Additional field to define the combinator

API changes

- New method MetatagSeparator::getCombinator()

Data model changes

- Additional setting "combinator"