State: Add timezones selectors functions

[retrofox]

This patch adds three state selectors and it's the first step of a set of PRs which improve the current implement of Timezones regarding the data approach, component, etc.

Testing

Beyond the app should work ok, you would run the selectors tests:

> npm run test-client client/state/selectors/test/index.js

NOTE: Selectors tests are already done (6814cff0d231fcab4233aa29c8bd9e1faa27013e), but they will be added in another PR here: #10941. The whole work related with the timezones improvements is here: #10788. You could take a look to see the trace of this process.

[matticbot]

Test live: https://calypso.live/?branch=update/timezones-state-selectors

[obenland]

Was ab1d23d5e5096cb16f64bc74952f9fceb3e28282 supposed to be part of this PR? The action types seem unrelated to the selectors.

Is there a reason you have the selector tests separated out in a different PR? Are they dependent on something other than these selectors?

[retrofox]

Was ab1d23d supposed to be part of this PR? The action types seem unrelated to the selectors.

Yes, It's a mistake. I should remove this commit from this PR

Is there a reason you have the selector tests separated out in a different PR? Are they dependent on something other than these selectors?

The only one reason is that these tests use a fixture file which is located into the state/timezone folder since it is used for testing either in these selectors as well as for the redux implementation.

This commit 6814cff0d231fcab4233aa29c8bd9e1faa27013e adds the selectors tests:

import { MANUAL_UTC_OFFSETS } from 'state/timezones/test/fixture';

I could create a transition commit if we consider necessary, though.

[retrofox]

@obenland In this PR #10934 I'm adding the fixture file to run the redux tests. I split up selectors and redux implementation in order to make shorter PRs.

So once #10934 is merged we will ale to add the selectors tests( WIP PR #10941)

[obenland]

Yeah, no need to get fancy, looks good from my end!

[retrofox]

Thanks @obenland for the review. Is it ready to merge?