Decouple the different contexts from each other (#24786)

Replace #16455

Close #21803

Mixing different Gitea contexts together causes some problems:

1. Unable to respond proper content when error occurs, eg: Web should
respond HTML while API should respond JSON
2. Unclear dependency, eg: it's unclear when Context is used in
APIContext, which fields should be initialized, which methods are
necessary.


To make things clear, this PR introduces a Base context, it only
provides basic Req/Resp/Data features.

This PR mainly moves code. There are still many legacy problems and
TODOs in code, leave unrelated changes to future PRs.

modules/translation/translation.go

@@ -38,10 +38,12 @@ type LangType struct {
 }
 
 var (
-	lock          *sync.RWMutex
+	lock *sync.RWMutex
+
+	allLangs   []*LangType
+	allLangMap map[string]*LangType
+
 	matcher       language.Matcher
-	allLangs      []*LangType
-	allLangMap    map[string]*LangType
 	supportedTags []language.Tag
 )
 
@@ -251,3 +253,9 @@ func (l *locale) PrettyNumber(v any) string {
 	}
 	return l.msgPrinter.Sprintf("%v", number.Decimal(v))
 }
+
+func init() {
+	// prepare a default matcher, especially for tests
+	supportedTags = []language.Tag{language.English}
+	matcher = language.NewMatcher(supportedTags)
+}