Add ability to change alignment of image captions

[SiobhyB]

Expected behavior

I expected to be able to centre captions I've added to images, as I'm able to in both the web editor and the Android app.

Actual behavior

It's not possible to centre captions. Even if you change the alignment of an image so that it's centred, captions remain aligned to the left.

Steps to reproduce the behavior

1. Navigate to the editor for a post in the app and upload a new image.
2. Tap on the image to add a caption.
3. Align the image to the centre.
4. Confirm the caption is still aligned to the left and attempt to centre it.

Tested on iPad Mini, iOS 11.7, WPiOS 12.1.3

Reported in 1762702-zen.

Related: #2310

[stale]

This issue has been marked as stale because:

• It has been inactive for the past year.
• It isn't in a project or a milestone.
• It hasn’t been labeled [Pri] Blocker, [Pri] High, or good first issue.

Please comment with an update if you believe this issue is still valid or if it can be closed. This issue will also be reviewed for validity and priority (cc @designsimply).