Ajax loader spinner

[e-kolpakov]

Description: this PR adds loader spinners to elements that trigger ajax request.

Testing instructions:

1. Loader spinner is added to initial DnD view (the one with "Loading drag and drop exercise")
2. Loader spinner is added to an item when it is dragged into a zone (both correct and incorrect), and removed from it when ajax request finishes

[itsjeyd]

@e-kolpakov Any specific advantage(s) of using padding-top instead of margin-top here? (Just asking because I'm curious :))

[e-kolpakov]

@itsjeyd the spinner is shown as if it is inside the text input, so it's just for the symmetry: both spinner wrapper and text input have the same padding.

[itsjeyd]

@e-kolpakov The spinners work well, and aside from the things I pointed out above the code LGTM as well. I like the idea of showing spinners right where users would be focusing their attention when dropping an item or submitting an additional answer! The only aspect I'm not entirely convinced about is how they behave for items that contain images instead of text -- it looks a bit weird to have the spinner displayed above the image. Do you think it would be possible to display the spinner on top of the image instead (sort of like an overlay)?

[mtyaka]

@e-kolpakov Instead of calling show/hide on the spinner div imperatively, the more correct virtual dom inspired approach would be to set a flag in the state (state.items[item_id].xhr_active = true), and then show/hide the spinner div inside view.js based on the xhr_active flag; something like:

h('div.spinner-wrapper', {style: {display: item.xhr_active ? 'block' : 'none'}}, [...])

In fact, we already have item.submitting_location and item.submitting_input that are supposed to be true when the xhr request that submits the location or input data is active, so at least in theory item.submitting_location || item.submitting_input should return true when any xhr requests related to the item are in progress and false otherwise.

[itsjeyd]

@e-kolpakov Looking great, thanks for the additional changes! 👍 from me once you remove that leftover comment. But since I don't know virtualdom that well, I'll let @mtyaka give the final go-ahead for merging.

[mtyaka]

@e-kolpakov Thanks for the changes, they look good! 👍 once you get the test passing.