Make switch use a random id if none is provided

[snide]

Fixes #748
Fixes #1027

Switches now generate a random id if none are passed (similar to how we do it in EuiFormRow. This should only happen when used on their own, since most other situations will usually generate one.

This seemed like the best solution since I could not require ids to be provided, otherwise it would spit out errors when EuiFormRow cloned ids down.

[chandlerprall]

Alternatively, you can mock the makeId function; having two tests, one where you pass id and one where id is generated, would provide the best coverage of the component's id logic.

[timroes]

This PR shows really, that someone should look into the whole htmlIdGenerator vs makeId mess. (#639, #637) and basically merge them again into one generator. Because htmlIdGenerator is the one we use all over Kibana to generate HTML ids, but within EUI some components seem to import makeId like this PR from the row component.

[cjcenizal]

Didn't look at the code but I tested this in VoiceOver in Chrome and it works as expected.

[snide]

Updated / rebased. I'll make an issue to update the test to be more intense, but merge this for now since it's been hanging for awhile.