PostShare block maintenance: localize, CSS migration and fixes, ESLint a11y fix

[jsnajdr]

Clean up usage of i18n-calypso
The ConnectionsList component doesn't use anything from the localize wrapper, which is therefore removed.
On the other hand, NoConnectionsNotice gets wrapped by localize and doesn't need to be passed the translate prop from the parent. It also gets its own file.

Migrate CSS styles to webpack
Fairly straighforward, no complications.

Fix some CSS glitches in NoConnectionsNotice
Before: notice the useless margin under the notice and unwanted border radius on the left:

After: both glitches get fixed:

Remove an ESLint-offending tabIndex prop from CalendarButton
The component ignores the prop anyway, so it's an easy win. Verify that tab navigation around the share block works as expected (after you connect a social account):

[matticbot]

Test live: https://calypso.live/?branch=migrate/post-share-moment-and-styles

[sgomes]

Code LGTM, and the test branch appears to work correctly.

Good work splitting out NoConnectionsNotice to facilitate code splitting! 👍