Add settings

[mds1]

Closes #60

This PR adds a Settings tab which:

• Enables wallet backup via file export and showing seed phrase. This is done in a manner identical to that of WalletBackupPrompt.vue
• Lets user change API endpoint. This required the following architecture changes:
  • config.json contains an array of possible network names, user-facing descriptions (these are basically just user-friendly titles to show in the settings dropdown), and the corresponding API base URL.
  • Wallet class now contains a apiEndpoint getter which returns the appropriate base URL based on the current network
  • All helpers in apiHelpers.ts were updated to no longer have a default API endpoint. This means the endpoint must be passed in by the wallet, which ensures they are always in sync and prevents accidentally using the default endpoint

[mds1]

Added persistence with local storage. Right now on login it's inefficient because it first updates the state on login, and again when it redirects to the balance page. For this PR I say we leave it as is and fix it when we work on caching in #63

Still waiting to hear about the select vs. text field for the endpoint

[mds1]

OK, the network selection settings now more closely match the mockups we were given (shown below). The only issue here is that for some reason the network requests are using a kaspasim: address instead of kaspatest:. I can't figure out why this is happening. @wildmolasses Do you have any insight on this?

It seems I also broke the tests by changing how networks are handled, so @wildmolasses I'll need your help there too