Replace button helper with standard button_tag

[jhawthorn]

This deprecates the button helper in favour of the standard rails button_tag.

The button takes a lot of positional arguments, one of which is deprecated. It's confusing because it looks like a helper that would ship with rails, but it is not.

I'm not sure if the deprecation is too aggressive, as button is used in a lot of extensions and custom store code, but it would be nice to eventually remove.

This generates slightly different markup. Instead of generating

<button class="button" type="submit">Sample text</button>

it generates

<button class="btn btn-primary">Sample text</button>

It should be fine to drop the type="submit", since that is the default, and our CSS button class is basically just an @extend of btn btn-primary.

The conversion was performed mostly with

sed -i "s/<%= button \\(t(['\".a-z_]*)\\) %>/<%= button_tag \1, class: 'btn btn-primary' %>/g" app/views/spree/admin/**/*.html.erb

[tvdeyen]

Very much appreciated 👏