Fix readme images.

[dessalines]

• Fixes Screenshots in Readme are broken #6587

[Nutomic]

I dont think its a good idea to use a third party website. Better to put them in .github folder so its directly under our control and doesnt require extra network requests.

[dessalines]

I think for these I'd rather keep them out of the git repos. They take up too much space, and have to belong to multiple repos anyway (lemmy, lemmy-ui, etc). postimgs isn't going away and should be okay.

[Nutomic]

The size of this repo is 38mb, and the images are 70kb so it really doesnt make a difference for the size. Having it directly in the repo means the screenshots also work offline, or if postimgs is unreachable for some reason.

Also the screenshots you used are from 0.19. Would be better to use 1.0 screenshots directly, otherwise we have to remember updating them later.

[dessalines]

Maybe we can just wait until the 1.0 release to add them to the git repo then. Because the 1.0 UI may still change, and then we'll have to keep adding image diffs to the git.