Style graph view buttons

[jamesamcl]

The graph view buttons use bootstrap classes, but I don't want to import the bootstrap css in OLS4 as it will probably break all of our existing CSS. Adding some rules for .btn etc should be enough to style the buttons.

They currently look like this (OLS4):

But should look like this (OLS3):

We could of course modify the graph view plugin itself, but as we have so far not had to I would like to avoid.