Rework spellchecking, add lint-spell (#29112)

Backport clean cherry-picks of
9c39f8515f
and
c7a21cbb0c
onto 1.21.

- Use maintained fork https://github.com/golangci/misspell
- Rename `mispell-check` to `lint-spell`, add `lint-spell-fix`
- Run `lint-spell` in separate actions step
- Lint more files, fix discovered issues
- Remove inaccurate and outdated info in docs (we do not need GOPATH for
tools anymore)

Maybe later we can add more spellchecking tools, but I have not found
any good ones yet.

docs/content/development/hacking-on-gitea.en-us.md

@@ -243,10 +243,10 @@ documentation using:
 make generate-swagger
 ```
 
-You should validate your generated Swagger file and spell-check it with:
+You should validate your generated Swagger file:
 
 ```bash
-make swagger-validate misspell-check
+make swagger-validate
 ```
 
 You should commit the changed swagger JSON file. The continuous integration