[Cashtab][Alias] Increase pending warning emphasis on confirmation modal

Summary:

• The title is now conditionally rendered: it's Confirm alias registration when the alias is available with no pending txs, otherwise it's a Warning title with icon.
• Modal body text also conditionally rendered based on existance of pending txs for this alias, with color scheme updated to use green for alias availability and the default alert color for the pending warning

Test Plan:

• Register a new alias and verify the confirmation dialog only has the confirmation message
• Register the same new alias and confirm the pending substrings and UI make match the following:

When alias is available with no pending registrations onchain

When alias is available with existing pending registrations from other users but not from this wallet

When alias is available with existing pending registrations which includes registrations from this wallet

Alias available, there are pending txs including from this wallet, and registration is on behalf of another address

Reviewers: bytesofman, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14725