There was no check for embedded images which made "copy text" paste
the siteURL instead of the actual image content.
This does not change the behavior of actually rendering the image
which is how other sites behave as well.
https://mattermost.atlassian.net/browse/MM-49627
```release-note
NONE
```
It was a good decision in hindsight to keep the public module as 0.x
because this would have been a breaking change again.
https://mattermost.atlassian.net/browse/MM-53032
```release-note
Changed the Go module path from github.com/mattermost/mattermost-server/server/v8 to github.com/mattermost/mattermost/server/v8.
For the public facing module, it's path is also changed from github.com/mattermost/mattermost-server/server/public to github.com/mattermost/mattermost/server/public
```
