Remove tweet-hover-light.less

The hover behaviour's now in tweet-light.less, and the file is no longer referred to.
Fix hover behaviour in search (light)

The coloured hovers didn't account for columns in strange places - for example, the search utility on the left of the screen. The hover colour now defaults to the non-coloured one, which is then overwritten for the colours when appropriate.
Remove unnecessary lines with hover (light)

Mentions and interactions use the same hover colour. Hence, they've been consolidated into a single section.