Moved ajax-loader.gif to achieve better caching

[Spedi]

Closes #9104

Moved ajax-loader.gif from the css folder to the images one, to achieve better caching, and updated all deprecated references to the file.

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@RayBB

[Spedi]

Hi @RayBB, I'm not sure why there's 3 commits to this PR, since I created a new branch just for the caching issue.
Please see the changes and let me know if I did something wrong (which is highly probable). Also I updated the references on 2 files but I can't see them in the commits here on Github, not sure why though.

[Spedi]

Got why I didn't see the update on references, didn't do git add .... noob error I suppose.
Nevertheless I can't understand why the other commits (Those about Issue #9135) are on this PR, maybe you can help me on elucidating the issue there.

[RayBB]

@Spedi you could just undo the changes related to old-style-lists and push that in a new commit.

[Spedi]

@RayBB Done that, it was an easy fix indeed.
Nonetheless, do you know why this stuff happened? I'm a newbie to the Git world so I may have messed up something in the process, it'll be very helpful to have some kind of feedback.
Thank you so much for your help and patience, really appreciate it, I'm learning a lot by working on this project! :)

[RayBB]

tested locally and is working great!
Just need staff to merge then I'll update my other PR :)

[mekarpeles]

The changes make sense, though I'm confused why we replaced the transparent version of the gif with one having a white background -- that may have some visual consequences.

Cc: @RayBB

[RayBB]

@mekarpeles idk why github show sit as not transparent but this is what it looks like in prod already.

The "transparent version" isn't used anywhere at all that's why we're removing it.
Personally, I think it would be quite nice to have a fancy svg loading indicator but I'll leave that to the design team to decide on :)