refs #7688
Adds an `uploads/icon/` endpoint to the api route to get a seperate entry point for blog icon validations. The blog icon validation will specifically check for images which have icon extensions (`.ico` & `.png`) and throw errors if:
- the icon file size is too big (>100kb)
- the icon is not a squaer
- the icon size is smaller than 32px
- the icon size is larger than 1000px
- the icon is not `.ico` or `.png` extension
TODOs for this PR:
- [X] get image dimensions
- [X] validate for image
- [X] size
- [X] form (must be square)
- [X] type
- [X] dimenstion (min 32px and max 1,000px)
- [X] return appropriate error messages
- [X] write tests
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TODOs for #7688:
- [X] Figure out, which favicon should be used (uploaded or default) -> #7713
- [ ] Serve and redirect the favicon for any browser requests, incl. redirects -> #7700 [WIP]
- [X] Upload favicon via `general/settings` and implement basic admin validations -> TryGhost/Ghost-Admin#397
- [X] Build server side validations -> this PR
