search_api_attachments 7.x-1.15

This is the Search API Attachments 1.15 release, Adding only new features :

1) Adding search_api_attachments_multiple_entities submodule
2) Adding search_api_attachments_links submodule
3) Support indexing when the index is type of multiple
4) Support of Search API Entity Translation in search_api_attachments_entityreference submodule

The commits merged since release 1.14:

* 2aaf73a Issue #2714531 by Yago Elias, nhamelin, geekygnr, izus: When the index is type of multiple, the file content is not indexed
* 1afd465 Issue #2596283 by izus: List search_api_attachments_multiple_entities in the root README file.
* 1150531 Issue #2596283 by frob, screon, izus, rovo, Grimreaper, kbrinner: No support for indexes on multiple entity types
* 1efd40b Issue #2955610 by izus: Add search_api_attachments_links to the toot README file to document it as a submodule.
* 3d0db4c Issue #2955610 by izus: Some fixes to search_api_attachments_links
* 0c80af2 Issue #2955610 by jackbravo, izus: Add a submodule to allow indexing link fields that point to a document like a PDF
* 26c6ed9 Issue #2617228 by Leksat, fjgarlin, Grimreaper, izus: Support for: entityreference field targeting a file entity, search_api_et module