[Cashtab] Enhance message display UI

Summary:

• Re-position the UI elements within the TxWrapper to maximize legibility of the OP_RETURN message and avoid overflows with Transaction properties seen in T1974.
• Verified across web view, mobile view and extension view.
• Adjusted message input placeholder text as per T1972.

Test Plan:

• npm start
• test with messages that have long words, short words, one liners...etc and ensure the message is displayed as per screenshots below.
• test with a message containing 150 consecutive characters of unbroken X (i.e. xxxxxxxxxxxxxxxxx...) and ensure message is contained within the TxWrapper UI.
• npm run extension.
• ensure same UI displays in extension mode.
• ensure same UI displays in firefox.
• use mobile phone to access the local Cashtab instance via bridged adapter and ensure same UI displays in actual mobile view.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10496