Add styles to the new messages drawer

[Info 3bits]

Moodle 3.6 introduces a new messages drawer.

It is using the default BS styles so it can re-stylised using default Adaptable styles.

Add settings in a future version.

[Info 3bits]

fix #973 Add Adaptable styles to the new messages drawer

→ <<cset 919ca4791915>>

[Info 3bits]

fix #973 Add Adaptable styles to the new messages drawer

→ <<cset 814225f14903>>

[Info 3bits]

Issue #970 was marked as a duplicate of this issue.