Problem
Copy and pasting article text into the caption input when it contains any links, references or templates leads to unexpected buggy behaviour such as not enabling the user to continue because the pasted text is not being registered in minimal input validation, and causing the device keyboard to be unusable.

Also see relevant related task: https://phabricator.wikimedia.org/T296114

Proposed solution
For iteration 1, we want to only allow pasting plain text (stripped of all formatting) into the caption input field.