Add min-height to text entry box of online parser

[hutchies]

At present, the CodeMirror text entry field on the right-hand side of the online test bed has no min-height set. This means that with complex grammars, a failed parse can result in error messages long enough that the text field shrinks to 0 and you can’t enter text. This can be fixed using browser dev-tools by finding the relevant element and setting a min-height of 100px - if this could be done in the code itself then it would make the test bed more robust.