Add spectral linter for Swagger (go-gitea#20321)

[spectral](https://github.com/stoplightio/spectral) lints
openapi/swagger files for mistakes of which it has identified a few and
which I've fixed.

I had to put it into `lint-frontend` because it depends on node_modules
so can not run on Drone during the backend target. I plan to refactor
these targets later to `lint-js` and `lint-go` so that they are
categorized based on the tool dependencies.

.spectral.yaml

@@ -0,0 +1,12 @@
+extends: [[spectral:oas, all]]
+
+rules:
+  info-contact: off
+  oas2-api-host: off
+  oas2-parameter-description: off
+  oas2-schema: off
+  oas2-valid-schema-example: off
+  openapi-tags: off
+  operation-description: off
+  operation-singular-tag: off
+  operation-tag-defined: off

Makefile

@@ -312,6 +312,7 @@ lint: lint-frontend lint-backend
 lint-frontend: node_modules
 	npx eslint --color --max-warnings=0 web_src/js build templates *.config.js docs/assets/js
 	npx stylelint --color --max-warnings=0 web_src/less
+	npx spectral lint -q -F hint $(SWAGGER_SPEC)
 
 .PHONY: lint-backend
 lint-backend: golangci-lint vet editorconfig-checker