Move hashtags to own entity

[florianschmidt1994]

Hashtags should be an own entity, because the use-case always dictates that hashtags are used for search and nothing else.
The information about an hashtag is also included in the text of a posting, so there is no need for it to be transferred as a list seperatly

PostingService.findByHashtag() --> HashtagService.findPostingByHashtag()

Applied principle: Try to avoid unnecessary one-to-many associations and instead use many-to-one with different repositories.