[iOS] Share to Zulip from other apps #54
Description
This issue is for implementing #52 on iOS. In other words, providing a "share extension" so as to let the user share content from other apps on Zulip.
(A lot of the work of implementing it will probably be separate between Android and iOS.)
This is a feature we actually don't have in the existing RN app: zulip/zulip-mobile#3277.