This PR does three things:
- First it moves the inline web app manifest into its own route `/manifest.json`
- Secondly, it add a setting `pwa.STANDALONE` that can be set to `true` if one wants users to be allowed to "install" forgejo as an pwa into their browser. This usually means an "install app" button, which essentially just creates an shortcut to use a single-tab window for browsing the app / forgejo.
- Thirdly since we have now an extra route, it checks if someone placed a `public/manifest.json` in forgejo's custom path; if yes, it's content is served instead. This allows more customization without the need on our side to completly implement every nuance of web app manifests.
This closes issue #2638
### Tests
- I added test coverage for Go changes...
- [x] in their respective `*_test.go` for unit tests.
### Documentation
- [x] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs/pulls/1669) to explain to Forgejo users how to use this change.
### Release notes
- [ ] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [x] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.
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## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Features
- [PR](https://codeberg.org/forgejo/forgejo/pulls/5384): <!--number 5384 --><!--line 0 --><!--description W2FsbG93IGZvcmdlam8gdG8gcnVuIGFzIGEgcHdhIHN0YW5kYWxvbmUgYXBwbGljYXRpb24gJiBvdmVycmlkZSBvZiB0aGUgd2ViYXBwIG1hbmlmZXN0Lmpzb24gdmlhIHRoZSBhIGN1c3RvbSBmaWxlIGluIGBwdWJsaWMvbWFuaWZlc3QuanNvbmBdKGh0dHBzOi8vY29kZWJlcmcub3JnL2Zvcmdlam8vZm9yZ2Vqby9wdWxscy81Mzg0KQ==-->[allow forgejo to run as a pwa standalone application & override of the webapp manifest.json via the a custom file in `public/manifest.json`](https://codeberg.org/forgejo/forgejo/pulls/5384)<!--description-->
<!--end release-notes-assistant-->
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/5384
Reviewed-by: Otto <otto@codeberg.org>
Reviewed-by: Lucas <sclu1034@noreply.codeberg.org>
Co-authored-by: Mai-Lapyst <mai-lapyst@noreply.codeberg.org>
Co-committed-by: Mai-Lapyst <mai-lapyst@noreply.codeberg.org>
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