[imorland]

Fixes: no issue filed.

Changes proposed in this pull request:

A forum can only set a favicon, which is stored as a 64px PNG and linked as a shortcut icon. There is no Apple touch icon or web app manifest, so phones show a screenshot or a letter when someone adds the forum to their home screen. .ico favicons are refused by the validator. A theme can't set any of its logos or icons from code.

• App icon: a new upload on the Appearance page, square and at least 512px. Flarum draws Apple's touch icon (180px, opaque on a background colour the admin picks), 192px and 512px manifest icons, and a 512px maskable icon padded to Android's safe zone. The page previews it as iPhones and Android show it, and changing the background colour redraws the icons from the kept source.
• Favicon: an upload also produces a 16/32/48px .ico, which the page links, and .ico files can be uploaded. Without a favicon, the app icon is used.
• Web app manifest at /manifest, served by Flarum so it works on any web server and stays on the forum's domain when assets are on S3 or a CDN. It opens the forum in the browser; Extend\Theme::manifest() lets an extension such as fof/pwa change that. /favicon.ico redirects to the icon for crawlers and feed readers.
• Extend\Theme can set every logo and icon: logo(), logoDarkMode(), emailLogo(), favicon() (with an optional SVG) and appIcon(). They go through the same pipeline as uploads, and the Appearance page shows them locked. logo(height, quality) from feat: store logos taller and at a higher quality #5309, not yet released, becomes logoSize().

Every size is drawn from the source, never from a smaller copy, and never enlarged. Icons are PNGs at the highest compression, and every file is stored with an explicit content type and a long-lived cache header, as names change whenever the image does.

Docs: flarum/docs#693

Reviewers should focus on:

• Flarum\Forum\Branding\Branding is the one place logo and icon URLs come from; ForumResource, the page head, emails, the standalone pages and discussion link favicons all ask it.
• ThemeImages draws a theme's images once, and again only when a file or option changes, keeping what it drew in a setting. A request compares file sizes and modification times once; an image that can't be drawn is logged and left out rather than retried on every request.
• New constructor arguments are optional and last, so existing subclasses keep working.
• fof/pwa links its own manifest after core's, so browsers use core's when there is an app icon until fof/pwa builds on Theme::manifest() instead (Build on core's web app manifest and app icon (Flarum 2.1) FriendsOfFlarum/pwa#36).
• Flarum's nginx config sends .ico requests straight to the filesystem, so /favicon.ico only reaches Flarum there with a config change in flarum/flarum ([2.x] Serve /favicon.ico through Flarum in the nginx config shipped with flarum/flarum #5320). Browsers use the page's links either way.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• Backend changes: tests are green (run composer test).
• Backend changes: tests have been added, or are not appropriate here.
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite). (No database changes.)
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.