Styling page loading

[vala]

In bd5a84b I introduced an overlay and a spinner over the main content area whenever a classic turbolinks page loading or form submission occurs.

This fixes the general unresponse feeling when the action takes some time to run.

Please @xana68 review the design if it's ok for you.

Note 1 :

For the spinner I used the following generator : http://preloaders.net/en/circular
Using the "Instagram loading" model, with a 32x32 size (This size is reflected in the para/admin/src/page-loading.sass stylesheet, so you'll need to change it if you change the spinner size)

Note 2 :

I created a stylesheets folder at para/admin/src which is required by the para/admin.sass main stylesheet, and added my code into a new para/admin/src/page-loading.sass. Is this the right way to add styles for you ?

[gabriel-cardoso]

👍 for the loader. No clue for Note 2

[xana68]

@vala I just put more opacity like .5 instead of .3 because i don't see the opacity very well.

Note 2 : Good ! so i put few files in it :)