Deploy gh pages to geoext3

[simonseyock]

This deploys the gh pages also to https://geoext.github.io/geoext3/ (not only to https://geoext.github.io/geoext). This ensures that any existing links to the api docs will still work in the future.

[marcjansen]

... alternatively redirect geoext3 to geoext? Probably not so easy for deep links.

[simonseyock]

We would need a redirect for every file. I added one for the index.html

[coveralls]

Pull Request Test Coverage Report for Build 864165267

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 81.182%

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Totals
Change from base Build 863985624:	0.0%
Covered Lines:	1376
Relevant Lines:	1618

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💛 - Coveralls