Add support for linking multiple Twitter accounts

[jeffpaul]

Is your enhancement related to a problem? Please describe.

Following on from the work in #229 to ensure compatibility with the latest Twitter API changes, we should look at adding support for maintaining multiple Twitter accounts. This also follows on from the discontinuing of Jetpack Social's Auto-sharing to Twitter and looking to mimic how that plugin handled multiple accounts so folks using Jetpack Social could migrate to Autoshare for Twitter and have a similar multi-account tweeting experience.

This will likely mean:

• updating the Settings to account for adding additional Twitter account connections
• update the sidebar panel to include a dropdown selection when multiple accounts are available

Designs

Rough mockup of how we might include a dropdown selection when there are multiple accounts registered:

Describe alternatives you've considered

We could look at providing some context of a "default" account that would be what's preset in the "Tweet from" dropdown, but getting this basic functionality released first and then gathering community feedback after might be a better use of time.

Code of Conduct

• I agree to follow this project's Code of Conduct

[jeffpaul]

After reviewing Jetpack Social's sidebar handling and discussing with @iamdharmesh, we've decided the following approach might be better than the original mockup shared in the issue description:

[jeffpaul]

Here's a sample mockup for how the Settings page could be laid out (ignore the terrible color mismatches and odd looking links/buttons/etc. and focus mostly on where things are laid out):

The "Edit" button could also be an action link instead and similarly expose the "Open connection settings" details from the current Settings experience, but in this case related to the specific account.