Add new Screen component and gradient themes

[kaloudis]

Description

• Adds gradient library react-native-linear-gradient
• gradientBackground property for themes
• new theme Popsicle, Nostrich, Desert, Orange Cream Soda, Red Metallic, Mint
• Refactor all pages to have sticky headers
• Remove shadows from Channels and Routing Events headers
• Add search toggle for Channels

This pull request is categorized as a:

• New feature
• Bug fix
• Code refactor
• Configuration change
• Locales update
• Quality assurance
• Other

Checklist

• I’ve run yarn run tsc and made sure my code compiles correctly
• I’ve run yarn run lint and made sure my code didn’t contain any problematic patterns
• I’ve run yarn run prettier and made sure my code is formatted correctly
• I’ve run yarn run test and made sure all of the tests pass

Testing

If you modified or added a utility file, did you add new unit tests?

• No, I’m a fool
• Yes
• N/A

I have tested this PR on the following platforms (please specify OS version and phone model/VM):

• Android
• iOS

I have tested this PR with the following types of nodes (please specify node version and API version where appropriate):

• LND (REST)
• LND (Lightning Node Connect)
• Core Lightning (c-lightning-REST)
• Core Lightning (Spark)
• Eclair
• LndHub

Locales

• I’ve added new locale text that requires translations
• I’m aware that new translations should be made on the Zeus Transfix page and not directly to this repo

Third Party Dependencies and Packages

• Contributors will need to run yarn after this PR is merged in
• 3rd party dependencies have been modified:
  • verify that package.json and yarn.lock have been properly updated
  • verify that dependencies are installed for both iOS and Android platforms

Other:

• Changes were made that require an update to the README
• Changes were made that require an update to onboarding