Enable App Extension Safe use of API's

[OskarGroth]

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.

Description

If a target (in our case, a general UI framework we use internally) uses the Require Only App-Extension-Safe API flag, it is unable to include SlackTextViewController because it refuses to build since it uses UIApplication sharedApplication in various places.

Reproducible in:

• [X] This is reproducible in the sample project.

Steps to reproduce:

1. Toggle Require Only App-Extension-Safe API to Yes.
2. Try building

Expected result:

Project succeeds to build

Actual result:

Project fails to build

Attachments:

e.g. Logs, screenshots, screencast, sample project, funny gif, etc.