Add notice to wallet route distinguishing Stacks Wallet

[markmhendrickson]

Currently, visitors to the Blockstack Browser's "Wallet" route are prone to confuse the Browser's BTC wallet shown there with the separate Stacks Wallet for STX tokens.

This issue entails adding a notice to the top of this route's view that alerts the user to the distinction and directs all users interested in the Stacks Wallet to its website.

[markmhendrickson]

@moxiegirl Could you please provide the content to provide in this notice with any relevant URLs?

[moxiegirl]

@markmhx please see the PR fix I made for this: #1965

[markmhendrickson]

@moxiegirl I think I saw in Slack that you may want to update this PR before release based on Ryan's feedback – is that the case?

[moxiegirl]

Yes.

[markmhendrickson]

@moxiegirl Cool. It'd be great to review those latest changes next Tuesday when we meet to discuss this change's inclusion in our next browser release.