Improved gallery cover lookup (#3391)

* Gallery cover lookup by regex in config.yml
* Added regex validation and an in-app manual entry
* Improved settings description + some translations
* Add changelog entry

internal/api/resolver_query_configuration.go

@@ -103,6 +103,7 @@ func makeConfigGeneralResult() *ConfigGeneralResult {
 		MaxTranscodeSize:             &maxTranscodeSize,
 		MaxStreamingTranscodeSize:    &maxStreamingTranscodeSize,
 		WriteImageThumbnails:         config.IsWriteImageThumbnails(),
+		GalleryCoverRegex:            config.GetGalleryCoverRegex(),
 		APIKey:                       config.GetAPIKey(),
 		Username:                     config.GetUsername(),
 		Password:                     config.GetPasswordHash(),