Lowercase the locale string used in the channel icon path locale placeholder

A manual uploaded image needs custom adjustment of the node name, if the locale is part of the image node name. Created node names are encoded by the node name codec and so don't container uppercase letters.

Reproduction:
1) upload custom icon for a locale, for example nl_NL.png. It has to matches the configured path template with placeholders [/hippo:configuration/hippo:frontend/cms/cms-static/channel-manager-perspective/channel-list/channelRegionIconPath, default value is /content/gallery/channels/${locale}.png/${locale}.png/hippogallery:original]
2) open the channel manager overview

Result: the custom icon is not getting rendered