Fix CSS specificity issue in extensions viewlet

[chiragjha1]

Addressing issue #238029
Fix for the issue with CSS specificity in Extensions Viewlet causing the last installed extension to overflow.

Problem:

• The CSS specificity targeting display property for the hidden container was too low, causing overflow of the last extension

Solution:

• Increased specificity of the .hidden class within .message-container so that when container is hidden, it stays hidden as intended.

Screenshots:

Testing: Have made sure that the last extension no longer overflows when the .hidden class is applied

[Sadullaev1]

nice