Update elasticsearch

- Build our own elasticsearch image based on 6.8.6 which includes
configuration files for german decompounder from https://github.com/uschindler/german-decompounder

- move elastichq to separate service and use the official image instead of including it as plugin in the elasticsearch image

- Adjust compounder filter config in papers to use the included hyphenation_decompounder.
See https://www.elastic.co/guide/en/elasticsearch/reference/6.8/analysis-compound-word-tokenfilter.html

- Remove obsolete "string" type in the mapping of papers, and replace it
with "text" or "keywords" accordingly. See https://www.elastic.co/blog/strings-are-dead-long-live-strings

Gemfile.lock

@@ -79,11 +79,11 @@ GEM
     elasticsearch-api (6.0.2)
       multi_json
     elasticsearch-dsl (0.1.5)
-    elasticsearch-model (5.0.0)
+    elasticsearch-model (6.1.0)
       activesupport (> 3)
       elasticsearch (> 1)
       hashie
-    elasticsearch-rails (5.0.2)
+    elasticsearch-rails (6.1.0)
     elasticsearch-transport (6.0.2)
       faraday
       multi_json
@@ -287,8 +287,8 @@ DEPENDENCIES
   database_cleaner
   elasticsearch (~> 6)
   elasticsearch-dsl
-  elasticsearch-model
-  elasticsearch-rails
+  elasticsearch-model (~> 6)
+  elasticsearch-rails (~> 6)
   factory_bot_rails
   faker
   foundation-rails (~> 5.5)