Rework 'make generate-images' (#12316)

* Rework 'make generate-images'

- Remove external dependencies and replace it with a node script that
  does does the same.
- Move detail removal from gitea-sm.png to favicon.png
- Remove favicon.ico and its generation, it is unused and we already serve
  favicon.png in its place.

Fixes: https://github.com/go-gitea/gitea/issues/12314

* use proper centering value for preserveAspectRatio

* fix lint

* use fabric

* better linting fix

* fix typo

* mention detail-remove class in docs

.eslintrc

@@ -33,6 +33,10 @@ overrides:
       worker: true
     rules:
       no-restricted-globals: [2, addEventListener, blur, close, closed, confirm, defaultStatus, defaultstatus, error, event, external, find, focus, frameElement, frames, history, innerHeight, innerWidth, isFinite, isNaN, length, location, locationbar, menubar, moveBy, moveTo, name, onblur, onerror, onfocus, onload, onresize, onunload, open, opener, opera, outerHeight, outerWidth, pageXOffset, pageYOffset, parent, print, removeEventListener, resizeBy, resizeTo, screen, screenLeft, screenTop, screenX, screenY, scroll, scrollbars, scrollBy, scrollTo, scrollX, scrollY, status, statusbar, stop, toolbar, top]
+  - files: ["build/generate-images.js"]
+    rules:
+      import/no-unresolved: [0]
+      import/no-extraneous-dependencies: [0]
 
 rules:
   accessor-pairs: [2]