Integrate Dex into Godcr

[dmigwi]

Created by: song50119

Integrate Dex into Godcr, will use modules from https://github.com/decred/dcrdex

• [x] On the UI create new menu item to navigate to dex page
• [x] Add modal to add new server
• [x] Add fee selector modal to choose DCR/BTC to pay the fee
• [ ] Add notifications modal
• [ ] Create mini trade form to swap dcr/btc coins(immediately sell/buy market book orders)
• [ ] Add Dex settings page, orders history page, wallet settings page

Notes

1. All the tasks have done at #722, but the code implemented need separate into smaller PRs.
2. Godcr will create BTC wallet and use it for DEX instead of DEX create and use BTC as it's own wallet, the rest of tasks will merge after this

(Issue originally authored by: @Kennedy Izuegbu in gitlab.com)