feature: detect clipboard for lightning invoice

[Free-FreeThinker]

Description

Auto detect clipboard for invoice when opening app, and ask the user if they'd like to send payment for the invoice if detected.

Motivation and Context

Address #329

How Has This Been Tested?

1. Put app into the background.
2. Copy a lightning invoice, and reopen the app, see the alert sheet successfully pops up.
3. Tap "Send", and successfully paid the invoice.
4. Copy some other random text, reopen the app, see no alert pops up.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.