Add character limit to connections

[ghost]

It is possible for some students to enter far too much text when adding a connection, which disrupts the layout of the board to which the connection is posted.

To see an example of this, navigate to an installation of linky which is backed by a live copy of linky.sql. Visit the Freedom of Religion theme on the Civil Liberties board (/boards/6/categories/29). One connection (the one which begins "In response to cyberbullying...") contains nearly 1500 characters, compared to an average of about 200 characters for the rest of the connections on the page.

Recommendation is to limit the number of characters in a connection description. 250 characters would be a reasonable limit.