Add hold over label on the left menu items when the menu is minimized

[kwunyeung]

Currently users may not know what those icons are if the left menu is minimized.

Suggest to have hold over label showing the menu labels after mouse cursor holding there for a few seconds.

Label Reference:

[kanniec826]

Please add your planning poker estimate with ZenHub @rachelhox

[nicoletting]

added hover label. view prototype here