[ImgBot] optimizes images

[SuperSandro2000]

*Total -- 239.51kb -> 210.99kb (11.91%)

/images/docs/create_PR.png -- 29.98kb -> 22.31kb (25.59%)
/images/docs/new_PR.png -- 11.84kb -> 9.53kb (19.54%)
/images/docs/commit_changes.png -- 23.20kb -> 19.21kb (17.17%)
/images/Logo.png -- 60.31kb -> 51.34kb (14.87%)
/images/default_profile.jpg -- 8.03kb -> 6.98kb (13.08%)
/images/docs/edit_file.png -- 12.15kb -> 11.45kb (5.8%)
/images/docs/choose_branch.png -- 14.48kb -> 13.70kb (5.34%)
/images/docs/create_fork.png -- 6.55kb -> 6.27kb (4.41%)
/images/docs/new_issue.png -- 7.36kb -> 7.04kb (4.31%)
/images/Logo_alpha.png -- 65.61kb -> 63.16kb (3.73%)

[GioBonvi]

@rowanthorpe can you please have a look at this when you have time?

I'd say a reduction of 28.52 kB over 239.51 kB is not worth it, but I'd like to hear another voice on the matter as I am not really knowleadgeble on the matter

[rowanthorpe]

@GioBonvi Sorry I can't properly "review" at the moment (i.e. check images are OK, etc) due to limited time (and for the next few weeks). I am just adding this comment instead and will leave it to you to override the need for review. It seems the commit was generated by ImgBot, so probably a more useful decision than whether to just accept this one-time modification or not is whether you want to integrate an image-squishing tool/toolset into your "build" workflow or not ("exporting" your original images without overwriting them - using something like Grunt, Gulp, Make, git-hooks, github "apps" like imgbot, etc), and if so then whether to do lossless image-crushing only, or lossy conversion on certain formats like jpg, etc. Doing that would ensure any modified and new images are also compressed. I haven't used ImgBot, but it looks like it is just a github-app-service wrapping commandline tools/toolsets like pngcrush, jpegoptim, imagemagick, etc. As for the question of whether ~10% size reduction is worth the overhead, I think the place where every kilobyte matters most is when sending images (i.e. profile-icons) in emails, but I think most of those are images retrieved by API rather than included in this repo anyway. As for the images in the repo, the screenshots are probably best kept as raster images (png), but I think it would be better to have the logos as vector images to save on space and to allow high-fidelity scaling (svg is broadly supported in browsers now). Whether you want to bother compressing (auto-compressing on "build") the screenshots is probably more about your personal preference and whether you can be bothered for the minor difference it would make.

[GioBonvi]

Thanks @rowanthorpe for the input, it was really insightful.

Personally, I don't think there is a lot to be gained from integrating ImgBot or similar tools in the build process at the moment, so I'll close this PR.
I'll keep this in mind as in the future we might need to reevaluate this choice (if we decide to ever implement something like #94 for example).

I like the idea of vectorizing the logo (which might benefit from a little bit of redesign as well): I'll open an issue for it.

Cheers