Allow zooming on pages

Summary:
Zooming on pages is currently disabled for reasons mentioned in rP953ab039acc0894e287304364920bc7cc4962cc0 from 2017.
This violates WCAG and is especially unhelpful on a mobile phone to see a larger version of an image file.
Thus allow zooming, crossing fingers that mobile platforms have reduced the unwanted side effects over the last seven years.

Closes T15945

Test Plan: Use Firefox on an Android phone in portrait orientation; Visit a Phorge task which has an image embedded; Click on the image comment; Pinch with fingers to zoom.

Reviewers: O1 Blessed Committers, 20after4

Reviewed By: O1 Blessed Committers, 20after4

Subscribers: 20after4, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15945

Differential Revision: https://we.phorge.it/D25827