Added confirmation dialog when expiring silences

[conr]

Closes #883

Added to silence list view and individual silence view.

See:

[stuartnelson3]

Thanks for the contribution! I assigned @w0rm to take a look, he should have some time soon.

Regarding the test failure: Once we're ready to merge, the last step is to run make build-all from the root dir. This compiles the elm code into javascript, and embeds the javascript into the alertmanager binary.

[conr]

@stuartnelson3 Strange it's failing as I ran make build-all as per the contribution.md for the Alertmanager ui.

[w0rm]

@Conorbro thanks for working on this issue!

I have two concerns:

1. do we really need to use a library for this or we can have our own dialog component?
2. showing a confirmation instead of a button or in a tooltip would be a better ux, because it doesn't block the whole ui

Because it's an improvement over what we have now, I think it can be merged, and then we could iterate on top.

[conr]

@w0rm

Yes I agree, I wasn't sure whether introducing another library would be a good idea or not.

It's a fairly simple library so we could probably implement something similar ourselves with a confirmation/tooltip as you have suggested in the future.