Add media size selection to confirmation screen.

Adding another issue to refine the scope of #4479 to just cover #4721.

Once #4721 has been merged it would be great to have a secondary option to choose the size of an image/video when the "Confirm size when sending" toggle has been disabled, along the lines of some of the designs seen in the original issue:

_Originally posted by @niquewoodhouse in https://github.com/vector-im/element-ios/issues/4479#issuecomment-886636826:_

After a brief design exploration of the issue we came to some solutions:

- Alter the confirm screen to include file size option

Would give people (harder) access to the option if they needed it, but make the flow easier for most users who don't want to edit size.


We did consider some other ideas, but they expanding the work more than we probably require at this stage.


Start by reviewing issue #4721 and the scope discussion in #4479, then inspect the existing confirmation screen in the iOS app. Compare its current flow with the selector designs linked here. Done means offering a secondary media-size choice when “Confirm size when sending” is disabled.

Written by the indexing model from the issue text.