fix(ui): changed layout of sidebar with transitions

[nikkothari22]

The sidebar was too "crowded". Too many icons - and always shown.

1. Expand and collapse buttons are only shown on hover and are moved to the right.
2. Add button is also moved to the right - and only shown on hover
3. Clicking on "Channels" or "Members" also toggles the list
4. Removed icon for pinned messages
5. Added animation for collapsing and expanding groups (will be used when we add channel groups as well)
6. Transition added for unread count badge

Before:

After: