Babel translating inner tags

Summary:
Add support for nesting inner tags and trimming.

Motivation / description:
The current extractor does not support nesting of inner tags. This creates problems when we have something like this "<translate> hello <b>beautiful</b> world</translate>". The message extraction gets prematurely cut off.

Futhermore, the extractor does not support trimming. This is important because angular-gettext trims the content and use that as the msgid. The goal is to get it working with angular-gettext, hence, we also need to trim our messages.

UX:
N/A

Outside Dependencies:
N/A

Doc Impact:
Update code comments to reflect this.