[Cashtab] Better component organization and routing for etokens

Summary:
Some cleanup after moving eTokens out of Home and onto their own screen. We shouldn't have src/components/Etokens and src/components/Tokens folders. Keep all the relevant components under src/components/Etokens

Tokens.js is confusingly named. This should be CreateToken.js, and its route should also be create-token.

Test Plan:
npm test

npm start, check tokens page, click create a token and note route

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15122