Don't hardcode URLs in Javascript

[dssjoblom]

PR for #2275 as requested.

[tvdeyen]

@dssjoblom thanks. Would you mind to add a spec and a proper PR title (maybe copy the commit message), because this will end up in the change log.

[dssjoblom]

@tvdeyen I changed the title, but I think writing a good spec for this is a bit beyond my capabilities. Ideally, the test would:

1. Mount Alchemy::Engine on a custom path e.g. /prefix
2. In Capybara, navigate to a specific page editor that contains a picture (e.g. prefix/admin/pages/1/edit)
3. In Capybara, click on the "change image" icon, select an image, and then press save

This test should work without any issues (the earlier code only worked if Alchemy was mounted on '/').

[tvdeyen]

@tvdeyen I changed the title, but I think writing a good spec for this is a bit beyond my capabilities. Ideally, the test would:

1. Mount Alchemy::Engine on a custom path e.g. /prefix
2. In Capybara, navigate to a specific page editor that contains a picture (e.g. prefix/admin/pages/1/edit)
3. In Capybara, click on the "change image" icon, select an image, and then press save

This test should work without any issues (the earlier code only worked if Alchemy was mounted on '/').

I was thinking about a view spec for the alchemy/admin/_routes partial. Everything else would be overkill.

[tvdeyen]

Since we do not have any specs for the view for other routes, I think it is fine to have any for this change.