fix(theme): change sponsor link class name to bypass adblock

[userquin]

closes #866

[brc-dd]

x-sponsor-link is also likely to be hidden by some ad blockers. Lets change it to something like sp-link.

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We should also track Carbon Ads (separately), currently this is what I get on my browser (using uBlock Origin):

I was hoping we could have something similar to Nuxt here.

There was an effort for this #348, but that PR went stale. The fix seemed quite straightforward. If you wish (and have some time to implement), we can have that here itself or maybe a separate PR by anyone. Either works for me.

[userquin]

upps, so we should ommit the sponsor key, fixing it...

[brc-dd]

Yeah, this one is fine. Using vp-sponsor-foo should also not cause issues with most of the ad-blockers, but that will probably conflict with our sponsors components.