Resolve #359 Optional Missing Alt-Text warning

[Lr-K]

Resolves https://github.com/Dimillian/IceCubesApp/issues/359

• Add toggle in settings to require alt text (default off)
• If setting is enabled, posting show an error if any attached media is missing alt text

Design decisions:

• Chose to surface an error instead of disabling posting, to make it obvious why the post is failing. A tooltip goes against current design decisions and isn't clear on all platforms.
• The setting is default off to minimise impact to existing users.

Toggle setting in content settings

Error displayed to user if attempting to post without all the necessary alt text (if setting is toggled on)

Tested on:

• iPadOS Simulator with photos and videos