Attached files and linked lecture attachments open with authorization error in browser

[julian-wls]

Description

As mentioned in #109, it is currently not possible to open resources like PDFs, etc, that were sent into the chat. #109 fixed the rendering of the links but did not fix the authorization issue.
When clicking on them, the user gets redirected to the browser, facing an authorization error instead of the requested resource.

Next steps

As a short-term solution, catch the link and open it manually, adding a cookie header, including the authorization token, would be a solid solution (see PostArtemisMarkdownTransformer and MarkdownText).
As a long-term solution, it would be good to have the files opened inside the app leading to a better usability:

• Open lecture content in the lecture unit it belongs to
• Open other files that were sent into the chat in a web view, maybe as modal bottom sheet.