UI: Add tags to documents

[mkruse83]

As a user I want to add tags to the document to help the search understand the contents of the text.

AC

• it must be possible to add tags to a document
• the tags must be stored in a field in the index
• it must be made sure that the user has to tag the document as either "Request" or "Offer"

Note
The tags request and offer will be used to identify which documents to use when asking questions. For example the user might ask a question about requirements. Those can be found in both types of documents - once as description on what is needed in requests, and once as descriptions on how the requirements will be fulfilled in the offer. In a later stage of the Bot he will ask what the user wants to chat about: Requests or Offers.