make AppName warning better

[pvinis]

Description

After #112, @lucasbento suggested some good changes:

• Have it appear only after the diff has been rendered
• Move it to be shown below the "Useful content" section
• Have the same width as the useful content section to maintain consistency
• Use "info" alert instead of "warning"
• Add a comment on it that the app name can be changed in the settings (this would happen in Feature: replace RnDiffApp with entered AppName #111 probably)

@Naturalclar, since you have worked on both of these, we would be very happy if you wanted to pick these ones up too 🙌

[lucasbento]

Just added a couple more points, @Naturalclar let us know what you think 🙂

[Naturalclar]

These points all sounds good! I can pick these up 👍

[lucasbento]

Done by @Naturalclar on #130 🙂