Steps to reproduce:

• go to a File page e.g. https://commons.wikimedia.beta.wmflabs.org/wiki/File:Chromefirefox_2014-01-10_12-38.png
• edit a caption
• click 'publish'
• while form is submitting, edit a depicts statement, and click 'publish' on that too
• you'll get an edit conflict

(easier way to reproduce this is to add a depicts tag but don't click publish, then switch tab and add a bunch of captions ... then quickly hit 'publish' for the captions, switch tabs, and hit 'publish' for depicts)

We want this:

• One way to prevent this is to disable submission of the depicts form while the captions form is submitting (and vice versa)
• Other way could be to have baseRevId live inside a Promise and find a way to chain API calls to it

Not 100% this is worthwhile, perhaps the edit conflict is not a problem?

Acceptance Criteria:

• not possible to get an edit conflict (or close this ticket as invalid)