Resolves: konveyor#1929

Functional changes:
1. document to be viewed is selected from the list (first action above
   the editor) or via URL
2. display language toggle based on languages supported by the document.
   For attachments the list consists of plain text and optionally a
   second language discovered based on file extension (YAML or JSON).
3. replace "merged" checkbox with an option in the select

Related refactorings:
1. split getTaskById() query into specialized queries:
   a) getTaskById() returning Promise<Task> for working with the object
   b) getTaskByIdAndFormat() returning Promise<string> for displaying
      the task as a formatted text
2. configure AnalysisDetails component to respond to 2 routes: existing
   details route and newly added /applications/:applicationId/analysis-details/:taskId/attachments/:attachmentId
3. persist merge mode in the URL as search param "merged"

Signed-off-by: Radoslaw Szwajkowski <rszwajko@redhat.com>