ui(style): consistent button styling

[theborakompanioni]

With #621 merged, the "create wallet" and "import wallet" look a bit different in styling, e.g. the back button and the button size and font sizes used. After this task, the flows should have a similar look-and-feel.

Also, in order to keep the UI consistent:

• use button size "lg" consistently instead of custom sizes
• use consistent button positioning (e.g. back button in "create wallet" and "import wallet")
• try to minimize custom button styling in individual components

But also look at and check the "main wallet view", "receive" "send", "earn" and "jam" components!

[theborakompanioni]

Blocked by #621