chore: move umd bundles to bundles directory

[devversion]

• Moves the generated .umd files to the bundles/ directory (as @angular/core)
• Uses a gulp plugin for rollup because its more clear and also allows us to easily minify & create sourcemaps in the future

Note: After this PR, sourcemaps are no longer generated for the umd bundles.

• This means that we also don't have sourcemaps for the demo-app (but those currently only map to the .js files)
• I locally played with it and created a special check to create sourcemaps when being in development mode (but it was kind of overkill and unnecessary as mentioned above)

@jelbourn I was not sure what's the best way to go here. We'd need find a clear way how we want to structure our gulp tasks (because having X similar tasks just for release feels wrong to me)

Closes #2308

[jelbourn]

LGTM

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