[feature] configurable maximum thumbnail dimensions (#4258)

- adds configuration for thumbnail maximum dimensions with warning on exceeding recommendations
- moves the media configuration vars into their own sub-struct
- replaces the configuration flag funcs with simple string consts

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4258
Reviewed-by: tobi <kipvandenbos@noreply.codeberg.org>
Co-authored-by: kim <grufwub@gmail.com>
Co-committed-by: kim <grufwub@gmail.com>

internal/router/template.go

@@ -53,7 +53,7 @@ func LoadTemplates(engine *gin.Engine) error {
 	if templateBaseDir == "" {
 		return gtserror.Newf(
 			"%s cannot be empty and must be a relative or absolute path",
-			config.WebTemplateBaseDirFlag(),
+			config.WebTemplateBaseDirFlag,
 		)
 	}