[eslint] relax ‘one-var’ to allow `let foo, bar`
[eslint] relax ‘one-var’ to allow `let foo, bar`

…but still force `let foo=1, bar=2` to be:
let foo=1
let bar=2

I don’t actually find any value in enforcing this rule at all and think
we could ignore it altogether but rather than advocating we remove it 
completely, this commit just makes it so you can combine uninitialized
variable declarations at the top.

I frequently see this in spec files where we do something like

let subject, result, whatever
//and then in a ‘beforeEach()’ we assign values to those
//and then in each test(..) they are used

What say ye all?

Test plan:
* look at follow-on commit to this that changes MessageStudentsSpec.js
* it should not have eslint errors about:
  `error  Split 'let' declarations into multiple statements`

Change-Id: I7aafac3a395bac221e9e9b3111a82d0af4671ddb
Reviewed-on: https://gerrit.instructure.com/134822
Tested-by: Jenkins
Reviewed-by: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>
Product-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
QA-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>

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