[Cashtab] Add transparent padding to eCash logo

Summary: As noted by @Klakurka in T1961, the eCash logo in prod has a small portion of the left and right edges cut off. This diff adds extra transaprent padding to both sides of the logo.

Test Plan:

• npm start
• cycle through the Wallet, eToken, Send and Setting menus and ensure top right eCash logo is displayed correctly
• manual resizing of browser width to ensure it is dynamically resized and look ok
• cross browser check in firefox
• this logo is not shown in extension mode, no testing needed there

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Subscribers: Klakurka

Differential Revision: https://reviews.bitcoinabc.org/D10480