mediawiki.ui: Add @borderRadius variable to be used for inputs and buttons

Bug: 70916
Change-Id: I2102033ae5faa16367ddd4cbb879fdc62b4ca0b0
checkbox.less: Use @borderRadius

Bug: 70916
Change-Id: Ia9b23b78c409744d7b4a34ff88f4231a96e15dad
Merge "mediawiki.ui: Add @borderRadius variable to be used for inputs and buttons"
Merge "checkbox.less: Use @borderRadius"