browse: Ensure display of valid utf-8 contents detected as binary
libmagic sometimes erroneously detects textual contents as binary while they are valid UTF-8 encoded files, so ensure they can be displayed in swh-web/browse.
Also add encoding information in the message printed in the content view when a content can not be displayed.
Related to #2082 (closed)
Test Plan
Turns out that this kind of content are present in the sample repositories used to populate the sample archive for the tests. So add a new test for that corner case.
Migrated from D2345 (view on Phabricator)