Add indexing of group wikis in index rake task

What does this MR do and why?

While implementing the GroupWiki Elasticsearch, we missed to index group wikis on gitlab:elastic:index. This MR creates two new rake tasks

• index_group_entities: Invokes index_epics and index_group_wiki
• index_group_wiki: Indexes all the ES enabled groups' wikis

gitlab:elastic:index task now calls index_group_entities

Screenshots or screen recordings

How to set up and validate locally

• Make sure Elasticsearch is enabled
• Create some Group wikis
• Run the following rake task

bundle exec rake gitlab:elastic:index

• Notice that in the console you should see this info Enqueuing Group level entities…, Indexing epics... and Indexing group wikis...
• Now run this:

bundle exec rake gitlab:elastic:index_group_entities

• Notice that in the console you should see this info Enqueuing Group level entities…, Indexing epics... and Indexing group wikis...
• Now run this:

bundle exec rake gitlab:elastic:index_group_wikis

• Notice that in the console you should see this info Indexing group wikis...
• Also make sure that the group wikis get populated in the Elastic

curl -XGET "http://localhost:9200/gitlab-development-wikis/_search" -H "kbn-xsrf: reporting" -H "Content-Type: application/json" -d'

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #336100 (closed)