Replace button_link_to with either button_to or link_to

[jhawthorn]

Similar to #2600 (and built on top of it to avoid conflicts)

This replaces all uses of our button_link_to helper with either button_to or link_to.

There's no reason for button_link_to to exist. Either we want a link (which we can style like a button easily if we want) or just a button. We should explicitly use the one we want given the situation.

All occurrences of button_link_to can be replaced with either button_to or link_to with class: 'btn btn-primary'. Both Rails built-in helpers can have a :method specified, and both are able to perform a GET request.

I've chosen to replace all button_link_to with a method with button_to, since that was the behaviour of the method, and the most appropriate use of a button and form tag.

I've replaced all button_link_to which did not specify a method (a GET request is desired) with link_to, since that was the previous behaviour and because this is what a link is.

(This should be rebased before merging)

[tvdeyen]

Thank you. That’s over due.

I appreciate the sensible distinction between links and buttons :ok-hand: