fix(avatar): use sha256 and inline the federated avatar lookup (#38843)

- Hash emails with sha256. Gravatar moved to sha256, and both it and
libravatar.org serve the same image for either hash.
- Drop `strk.kbt.io/projects/go/libravatar` for a 46 line inline SRV
lookup. It could not bound or cancel its DNS query and panicked on an
unexpected resolver error. The replacement carries the request context
and a 3s timeout.
- Fix federated avatars querying DNS for every avatar on every render.
`loadAvatarSetting` compared a cache field that was never assigned, so
each call rebuilt the resolver and dropped its cache. That cache is
gone, both settings are read where they are used.
- Migration 348 recreates `email_hash` with a 64 char hash column and a
`hash_type` column, so a later algorithm change can tell old rows apart.
The MD5 rows are unreachable and their `UNIQUE` email index would reject
the SHA256 replacements.
- Fix a re-saved avatar form replacing an uploaded avatar with a random
one.
- Remove the `duoshuo` `GRAVATAR_SOURCE` alias, that service shut down
in 2017.
- Remove dead i18n key.

Fixes: https://github.com/go-gitea/gitea/issues/34284
Fixes: https://github.com/go-gitea/gitea/issues/28110
Docs: https://gitea.com/gitea/docs/pulls/499
Signed-off-by: silverwind <me@silverwind.io>

services/forms/user_form.go

@@ -239,18 +239,13 @@ func (f *UpdateLanguageForm) Validate(req *http.Request, errs binding.Errors) bi
 	return middleware.Validate(ctx, errs, f)
 }
 
-// Avatar types
-const (
-	AvatarLocal  string = "local"
-	AvatarByMail string = "bymail"
-)
+const AvatarLocal = "local" // the AvatarForm.Source value that selects an uploaded avatar
 
 // AvatarForm form for changing avatar
 type AvatarForm struct {
-	Source      string
-	Avatar      *multipart.FileHeader
-	Gravatar    string `binding:"OmitEmpty;Email;MaxSize(254)"`
-	Federavatar bool
+	Source   string
+	Avatar   *multipart.FileHeader
+	Gravatar string `binding:"OmitEmpty;Email;MaxSize(254)"`
 }
 
 // Validate validates the fields