Don't export angular expressions, allow export attributes like title …

[sWitched-0FF]

If we add to WebpackAngularGettext.loader.attributes = ["titles", "placeholder"] script add in export.pot tagHtml instead of title attribute value. In addition skip export angular expressions without any text.

[ngehlert]

there are merge conflicts. also can you try to elaborate for what usecase you need this change? looks like just prevention to use it "wrong". Or do you have a real usecase where <div translate>{{inner}}</div> actually make sense?