Support rebuilding of the synonym resource

Problem/Motivation

If the synonym resource gets corrupted there is no way to rebuild the registry, this for example might occur if the managed resource can't write the data to file so it only exists in memory. If SOLR gets restarted then the synonyms are unknown by the index, and the only way to get them back is to remove all synonyms from Drupal and re-add them one by one.

Proposed resolution

Add an additional "rebuild synonym list" action.

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A