software/theia: update to 1.9.0 and update extensions

Update to 1.9.0 ( https://github.com/eclipse-theia/theia/blob/v1.9.0/CHANGELOG.md#v190---16122020 )

Update extensions to newer version and to the URLs from openvsx instead of
github, since openvsx prevent maintainers to modify releases.

Also use buildout directly to download extensions, so that we verify signatures and have possibility to use cache

See merge request !873
6 jobs for master in 0 seconds
Status Job ID Name Coverage
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passed SlapOS.SlapRunner.ResilienceTest-Master

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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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passed SlapOS.SlapRunner.ResilienceTest-Master

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passed SlapOS.SlapRunner.ResilienceTest-Master

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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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