Add guidance for highlight length

**Is your feature request related to a problem? Please describe.**
Currently there is no mechanism to stop someone selecting any amount of characters, for example a whole article, which may be relevant or may be just lack of preciseness.

**Describe the solution you'd like**
When someone highlights beyond x (to be defined) number of characters, a box appears that informs them that they have highlighted a lot of text. It should offer advice on how to break the large text up into smaller more succinct highlights and check they really want to highlight this much.

One sentence is around 75-100 characters on average so approximately 300 but this is open to discussion.

**Additional context**
Add any other context or screenshots about the feature request here.