Streamline onboarding flow

[grod220]

In the attempt to simplify onboarding and reduce complexity, we will be making the following changes:

• Remove wallet birthday from UI: remove wallet birthday height from onboarding flow #221. Keep the underlying mechanics of sync speed ups for new wallets.
• Do not require users to choose the length of seed phrase (just always generate 12 words). For importing seedphrase, still allow 24 words to be imported.
• Do not require users to choose an RPC and frontend (automatically choose for them)
• Do not require users to choose a numeraire (automatically choose all for them)

A new user flow should look like:

• Click install
• Click "Create account"
• Back up seed phrase somewhere
• Do challenge for seed phrase
• Done

Edit: Note this as well #224 (comment)